add instruction for contriubtions

CONTRIBUTING.md

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+## How to contribute:
+
+1. Fork and clone this repo. Install dependencies by running `npm i` (you need to have node.js).
+2. Open ./palettes/palettes.html in browser to see the available colors for the theme you're interested in.
+3. Open ./palettes/theme_name/theme_name.js to see how the aliases are mapped.
+4. Open ./syntax-highlighting-rules and edit the json files. Use aliases or color names, not color values.
+5. run `npm run visualize` to see what it looks like in VSCode, no need to refresh. (Back up your settings.json first just in case, although you shouldn't need to.)
+6. Keep tweaking the rules, maybe create new aliases along the way, submit a pull request when satisfied.
+7. Delete the `"editor.tokenColorCustomizations"` entry in your settings.json afterwards.