docs: basic README

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I6f6a498f79871c745c8bbb87a45171d66a6a6964

README.md

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+# eh - Ergonomic Nix CLI Helper
+
+[intelligent error handling]: #auto-retry-with-environment-variables
+[automatic fixes]: #hash-auto-fix
+
+`eh` is a multicall [^1] (think Busybox) CLI tool that provides _ergonomic_
+shortcuts for common Nix commands with [intelligent error handling], and more
+importantly, [automatic fixes] for common Nix errors.
+
+[^1]: See [aliases](#shell-aliases) section.
+
+## Features
+
+We provide a very tiny binary with a limited set of features. The most critical
+features are **automatic hash fixes** and **auto-retry with appropriate
+environment variables**.
+
+### Auto-Retry with Environment Variables
+
+When a package fails to build due to licensing, security, stability, or whatever
+reason provided by Nixpkgs, `eh` equivalent of Nix commands retry automatically
+with the appropriate environment variables. While this is called automatic
+_retry_, we collect information about the packages from the `meta` field instead
+of building the package. The following variables are supported:
+
+- **Unfree packages**: Sets `NIXPKGS_ALLOW_UNFREE=1`
+- **Insecure packages**: Sets `NIXPKGS_ALLOW_INSECURE=1`
+- **Broken packages**: Sets `NIXPKGS_ALLOW_BROKEN=1`
+
+### Hash Auto-Fix
+
+When a hash mismatch is detected in the underlying `nix build`, `eh` can
+automatically update the old, broken hash with a new and correct one _directly
+in the source file_.
+
+## Shell Aliases
+
+By default, you may run the `eh` binary akin to Nix with a nicer interface. The
+supported Nix commands, i.e., nix `build`, `shell`, `run` and `develop` become
+`eh build`, `eh shell`, `eh run` and `eh develop`. However, it is possible to
+symlink the `eh` binary to `nb`, `ns`, `nr`, and `nd` to invoke a specific
+feature. For example, `nb` will act as `eh build` and `nr` will be `eh run`.
+
+One special example is `eh update`, which is aliases to `nu`, that handles
+interactive Nix flake updates. It is special in the sense that the usage is
+entirely different from its Nix counterpart, where you get to _interactively_
+pick which inputs to update.
+
+After enabling shell aliases via the NixOS module or Home Manager, you can use:
+
+```bash
+ns nixpkgs#hello           # equivalent to: nix shell nixpkgs#hello
+nr nixpkgs#cowsay "Hello!" # nix run nixpkgs#cowsay
+nb .#myPackage             # nix build .#myPackage
+nu                         # nix flake update
+```
+
+## License
+
+<!--markdownlint-disable MD059-->
+
+This project is made available under Mozilla Public License (MPL) version 2.0.
+See [LICENSE](LICENSE) for more details on the exact conditions. An online copy
+is provided [here](https://www.mozilla.org/en-US/MPL/2.0/).
+
+<!--markdownlint-enable MD059-->