Update readme with new procedures

Makefile

@@ -0,0 +1,6 @@
+.PHONY: default
+
+default:
+	cd tools/ && python gen.py
+	cd tools/ && python -m screenshot_gen
+	cd tools/ && python generate_screenshots_readme.py

README.md

@@ -84,7 +84,9 @@ There are 3 ways to install an iTerm theme:
 
 ### How to add new theme
 
-Have a great iTerm theme? Send it to me via a Pull Request!
+Have a great theme? Send it to me via a Pull Request!
+
+#### Have an iTerm theme?
 
 1. Get your theme's`.itermcolors` file.
    - Launch iTerm 2
@@ -95,17 +97,33 @@ Have a great iTerm theme? Send it to me via a Pull Request!
    - Save the .itermcolors file
 2. Put your theme file into `/schemes/`
    - `mv <your-itermcolors-file> schemes/`
-3. Generate other formats for your theme using the `gen.py` script.
+
+#### Have a theme in another format?
+
+1. Convert it to the YAML format specified in `yaml/README.md`.
+   This is an extension of the format supported by the [Gogh](https://github.com/Gogh-Co/Gogh/) project.
+   * If it helps, you can use `tools/kitty_to_yaml.py` and `tools/ghostty_to_yaml.py`.
+     These tools accept configuration file streamed into stdin, and output a YAML fragment to stdout. 
+2. Put the YAML file in `yaml/`, with the `.yml` extension.
+
+#### Create derived versions
+
+If you have `make` installed, steps 1 to 4 can be run with `make` from the root of the repository.
+
+1. Generate other formats for your theme using the `gen.py` script.
    - `cd tools/ && python3 gen.py` OR
    - `cd tools/ && ./gen.py`
-4. If you only want to generate files for your theme, you can specify this with the `-s` flag.
+2. If you only want to generate files for your theme, you can specify this with the `-s` flag.
    - `./gen.py -s Dracula`
-5. Generate a screenshot of your theme using the `screenshot_gen` tool.
+3. Generate a screenshot of your theme using the `screenshot_gen` tool.
    - `cd tools/ && python -m screenshot_gen`. This will generate new screenshots where they are missing.
    - If you have `oxipng` or `zopflipng` installed, the screenshot will be optimized for you.
-6. Run `generate_screenshots_readme.py` to include your theme's screenshot in the `screenshots/README.md` file:
+4. Run `generate_screenshots_readme.py` to include your theme's screenshot in the `screenshots/README.md` file:
    - `cd tools/ && python generate_screenshots_readme.py`
-7. Update `README.md` to include your theme and screenshot. Also update `CREDITS.md` to credit yourself for your contribution.
+
+#### Add your theme to the README
+
+1. Update `README.md` to include your theme and screenshot. Also update `CREDITS.md` to credit yourself for your contribution.
 
 ### How to add new template