docs: add development instructions

README.md

@@ -11,3 +11,45 @@ It then maps the original World to the changed World, with our desired IO action
 > From the second perspective, an IO action transforms the whole world. IO actions are actually pure, because they receive a unique world as an argument and then return the changed world.
 
 See [this](https://lean-lang.org/functional_programming_in_lean/monads/io.html) for more information.
+
+## Hacking on suntheme
+
+It's trivially easy to get started with suntheme development thanks to [Nix](https://nixos.org/), the purely functional package manager.
+Naturally, we leverage it as our primary package manager, for both Hackage packages and development tools like language servers and the like.
+
+First, install Nix through your preferred avenue. If unsure, we recommend [the Determinate Nix Installer](https://github.com/DeterminateSystems/nix-installer).
+Make sure flakes and nix-command are enabled (the Determinate Installer will enable them by default).
+
+Once you have `nix`, simply type
+
+```bash
+nix develop
+```
+
+Say yes to any prompts asking you to allow substituters or trust public keys.
+Nix will fetch all of the required packages, such as GHC and Hackage dependencies.
+Additionally, you will have access to `hlint` and the `haskell-language-server`.
+
+To create a build, type
+
+```bash
+nix build
+```
+
+A binary will be produced in `result/bin/suntheme`.
+
+To make `suntheme` available in the shell without outputting to `result`, use
+
+```bash
+nix shell
+```
+
+This will build a binary just like `nix build` but add it temporarily to the PATH, so you can just type `suntheme`.
+
+To build and run `suntheme` immediately without adding it to the PATH, use
+
+```bash
+nix run
+```
+
+This will build a binary just like `nix shell` but immediately execute it.