doc: update configuration docs on custom plugins

docs/manual/configuring/custom-plugins/legacy-method.md

@@ -0,0 +1,35 @@
+# Legacy Method {#sec-legacy-method}
+
+Prior to version 0.5, the method of adding new plugins was adding the plugin
+package to `vim.startPlugins` and add its configuration as a DAG under one of
+`vim.configRC` or `vim.luaConfigRC`. Users who have not yet updated to 0.5, or
+prefer a more hands-on approach may use the old method where the load order of
+the plugins is determined by DAGs.
+
+## Adding plugins {#sec-adding-plugins}
+
+To add a plugin to **nvf**'s runtime, you may add it
+
+```nix
+{pkgs, ...}: {
+  # add a package from nixpkgs to startPlugins
+  vim.startPlugins = [
+    pkgs.vimPlugins.aerial-nvim  ];
+}
+```
+
+And to configure the added plugin, you can use the `luaConfigRC` option to
+provide configuration as a DAG using the **nvf** extended library.
+
+```nix
+{inputs, ...}: let
+  # assuming you have an input called nvf pointing at the nvf repository
+  inherit (inputs.nvf.lib.nvim.dag) entryAnywhere;
+in {
+  vim.luaConfigRC.aerial-nvim= entryAnywhere ''
+    require('aerial').setup {
+      -- your configuration here
+    }
+  '';
+}
+```