docs: add keymapping adding documentation

.github/CONTRIBUTING.md

@@ -23,3 +23,153 @@ The contribution process is mostly documented in the [pull request template](.gi
 This project does not quite have a code of conduct yet. And to be honest, I'm not sure if I want one. I'm not expecting this project to be a hotbed of activity, but I do want to make sure that everyone who does contribute feels welcome and safe. As such, I will do my best to make sure that those who distrupt the project are dealt with swiftly and appropriately.
 
 If you feel that you are not being treated with respect, please contact me directly.
+
+## Custom key mappings support for a plugin
+
+To add custom keymappings to a plugin, a couple of helper functions are available in the project.
+
+To set a mapping, you should define it on `vim.maps.<mode>`.
+The available modes are:
+
+- normal
+- insert
+- select
+- visual
+- terminal
+- normalVisualOp
+- visualOnly
+- operator
+- insertCommand
+- lang
+- command
+
+An example, simple keybinding, can look like this:
+
+```nix
+
+{
+  vim.maps.normal = {
+    "<leader>wq" = {
+      action = ":wq<CR>";
+      silent = true;
+      desc = "Save file and quit";
+    };
+  };
+}
+```
+
+There are many settings available in the options. Please refer to [the documentation](https://notashelf.github.io/neovim-flake/options.html#opt-vim.maps.command._name_.action) to see a list of them.
+
+neovim-flake provides a list of helper commands, so that you don't have to write the mapping attribute sets every time:
+
+`mkBinding = key: action: desc:` - makes a basic binding, with `silent` set to true.  
+`mkExprBinding = key: action: desc:` - makes an expression binding, with `lua`, `silent`, and `expr` set to true.  
+`mkLuaBinding = key: action: desc:` - makes an expression binding, with `lua`, and `silent` set to true.  
+Note - the lua in these bindings is _actual_ lua, not pasted into a `:lua`.  
+Therefore, you either pass in a function like `require('someplugin').some_function`, without actually calling it,
+or you define your own function, like `function() require('someplugin').some_function() end`.
+
+Additionally, to not have to repeat the descriptions, there's another utility function with its own set of functions:
+
+```nix
+# Utility function that takes two attrsets:
+# { someKey = "some_value" } and
+# { someKey = { description = "Some Description"; }; }
+# and merges them into
+# { someKey = { value = "some_value"; description = "Some Description"; }; }
+
+addDescriptionsToMappings = actualMappings: mappingDefinitions:
+```
+
+This function can be used in combination with the same mkBinding functions as above, except they only take two arguments - `binding` and `action`, and have different names.  
+`mkSetBinding = binding: action:` - makes a basic binding, with `silent` set to true.  
+`mkSetExprBinding = binding: action:` - makes an expression binding, with `lua`, `silent`, and `expr` set to true.  
+`mkSetLuaBinding = binding: action:` - makes an expression binding, with `lua`, and `silent` set to true.
+
+You can read the source code of some modules to see them in action, but their usage should look something like this:
+
+```nix
+# plugindefinition.nix
+{lib, ...}:
+with lib; {
+  options.vim.plugin = {
+    enable = mkEnableOption "Enable plugin";
+
+    # Mappings should always be inside an attrset called mappings
+    mappings = {
+      # mkMappingOption is a helper function from lib,
+      # that takes a description (which will also appear in which-key),
+      # and a default mapping (which can be null)
+      toggleCurrentLine = mkMappingOption "Toggle current line comment" "gcc";
+      toggleCurrentBlock = mkMappingOption "Toggle current block comment" "gbc";
+
+      toggleOpLeaderLine = mkMappingOption "Toggle line comment" "gc";
+      toggleOpLeaderBlock = mkMappingOption "Toggle block comment" "gb";
+
+      toggleSelectedLine = mkMappingOption "Toggle selected comment" "gc";
+      toggleSelectedBlock = mkMappingOption "Toggle selected block" "gb";
+    };
+  };
+}
+```
+
+```nix
+# config.nix
+{
+  pkgs,
+  config,
+  lib,
+  ...
+}:
+with lib;
+with builtins; let
+  cfg = config.vim.plugin;
+  self = import ./plugindefinition.nix {inherit lib;};
+  mappingDefinitions = self.options.vim.plugin;
+
+  # addDescriptionsToMappings is a helper function from lib,
+  # that merges mapping values and their descriptions
+  # into one nice attribute set
+  mappings = addDescriptionsToMappings cfg.mappings mappingDefinitions;
+in {
+  config = mkIf (cfg.enable) {
+    # ...
+
+    vim.maps.normal = mkMerge [
+      # mkSetBinding is another helper function from lib,
+      # that actually adds the mapping with a description.
+      (mkSetBinding mappings.findFiles "<cmd> Telescope find_files<CR>")
+      (mkSetBinding mappings.liveGrep "<cmd> Telescope live_grep<CR>")
+      (mkSetBinding mappings.buffers "<cmd> Telescope buffers<CR>")
+      (mkSetBinding mappings.helpTags "<cmd> Telescope help_tags<CR>")
+      (mkSetBinding mappings.open "<cmd> Telescope<CR>")
+
+      (mkSetBinding mappings.gitCommits "<cmd> Telescope git_commits<CR>")
+      (mkSetBinding mappings.gitBufferCommits "<cmd> Telescope git_bcommits<CR>")
+      (mkSetBinding mappings.gitBranches "<cmd> Telescope git_branches<CR>")
+      (mkSetBinding mappings.gitStatus "<cmd> Telescope git_status<CR>")
+      (mkSetBinding mappings.gitStash "<cmd> Telescope git_stash<CR>")
+
+      (mkIf config.vim.lsp.enable (mkMerge [
+        (mkSetBinding mappings.lspDocumentSymbols "<cmd> Telescope lsp_document_symbols<CR>")
+        (mkSetBinding mappings.lspWorkspaceSymbols "<cmd> Telescope lsp_workspace_symbols<CR>")
+
+        (mkSetBinding mappings.lspReferences "<cmd> Telescope lsp_references<CR>")
+        (mkSetBinding mappings.lspImplementations "<cmd> Telescope lsp_implementations<CR>")
+        (mkSetBinding mappings.lspDefinitions "<cmd> Telescope lsp_definitions<CR>")
+        (mkSetBinding mappings.lspTypeDefinitions "<cmd> Telescope lsp_type_definitions<CR>")
+        (mkSetBinding mappings.diagnostics "<cmd> Telescope diagnostics<CR>")
+      ]))
+
+      (
+        mkIf config.vim.treesitter.enable
+        (mkSetBinding mappings.treesitter "<cmd> Telescope treesitter<CR>")
+      )
+    ];
+
+    # ...
+  };
+}
+```
+
+If you have come across a plugin that has an API that doesn't seem to easily allow custom keybindings, don't be scared to implement a draft PR. We'll help you get it done.