chore(build): auto-generate vimdoc

doc/astrotheme.txt

@@ -310,6 +310,31 @@ CONTRIBUTING                                         *astrotheme-contributing*
 If you plan to contribute, please check the contribution guidelines
 <https://github.com/AstroNvim/.github/blob/main/CONTRIBUTING.md> first.
 
+When contributing to the extras, we use a templating system for automatically
+generating extra files for all of the different color palettes. Here are some
+steps to get started:
+
+1. Create a file like `lua/astrotheme/extras/cool-app.lua`
+
+
+2. Add the name and output file extension to the `extras` table in
+`lua/astrotheme/extra/init.lua`
+
+
+3. Generate the new extra theme files by running the following command in the root
+of the repository:
+
+>sh
+    make extras
+<
+
+
+4. Verify the new templates are working by checking the generated files in the
+`extras/` directory. Please **DO NOT** commit them, as they get automatically
+built by the CI.
+
+
+
 
 ==============================================================================
 1. Lua API                                                *astrotheme-lua-api*