doc: add contribution docs on adding plugins

.github/CONTRIBUTING.md

@@ -237,4 +237,36 @@ in {
 }
 ```
 
+#### Adding Plugins
+
+To add a new (neo)vim plugin, add the source url in the inputs section of `flake.nix`
+
+```nix
+{
+    inputs = {
+        # ...
+        neodev-nvim = {
+            url = "github:folke/neodev.nvim";
+            flake = false;
+        };
+    };
+}
+```
+
+Then add the name of the plugin into the `availablePlugins` variable in `lib/types/plugins.nix`:
+
+```nix
+# ...
+availablePlugins = [
+    # ...
+    "neodev-nvim"
+];
+```
+
+You can now reference this plugin using its string name:
+
+```nix
+config.vim.startPlugins = ["neodev-nvim"];
+```
+
 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.