Update CONTRIBUTING.md

Author: owen-m1

CONTRIBUTING.md

@@ -10,5 +10,17 @@
 
 ### Pull Request
 
- 1. Only into [master](https://github.com/SortableJS/Sortable/tree/master/)-branch.
- 2. Do not modify `Sortable.min.js`, this will be compiled before each release.
+ 1. Only request to merge with the [master](https://github.com/SortableJS/Sortable/tree/master/)-branch.
+ 2. Only modify source files, **do not commit the resulting build**
+
+### Setup
+
+ 1. Fork the repo on [github](https://github.com)
+ 2. Clone locally
+ 3. Run `npm i` in the local repo
+
+### Building
+
+ - For development, build the `./Sortable.js` file using the command `npm run build:umd:watch`
+ - To build everything and minify it, run `npm run build`
+ - Do not commit the resulting builds in any pull request – they will be generated at release