Merge pull request #798 from kit-ty-kate/contributing

Add a basic CONTRIBUTING.md file

Author: smorimoto

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+### Developement setup
+
+Requirements:
+
+- [Node.js](https://nodejs.org)
+- [Corepack](https://nodejs.org/api/corepack.html)
+  - Shipped with Node.js, however on some distributions it is not distributed, in which case install it through npm using: `npm install --global corepack`
+
+To regenerated the compiled JavaScript files in `dist/`, call:
+
+```
+corepack enable
+yarn install --immutable
+yarn build
+```