Create README.md and describe how to add a new theme (#442)

* Create README.md and describe how to add a new theme

* Correct adding theme step description

* Update README.md

README.md

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+Adding a Mint-Y theme
+=====================
+
+1. Add new colors to `constants.py`
+2. Add new hue to `colorize.py`
+3. Add the new color name to the lists in `update-variations.py`
+4. Run the Python script `update-variations.py` for the new color (e.g. `./update-variations.py Red`). 
+This will generate new assets in `src/Mint-Y/variations/`.
+5. Manually create a new directory for the new theme in `files/usr/share/themes`. It's best to copy an existing one as a template.
+
+    In there:
+    - Adapt the theme name in `index.theme`.
+    - Create new `thumbnail.png` images.
+6. Run the Python script `generate-themes.py`. This will create the themes in the local directory `usr/share/themes`.
+7. Test the themes by copying them to `~/.themes` and applying them.
+8. In Git commit and push all newly created files to a new branch.