Add Contributing instrictions to README, Setup grunt

Add some instructions for newcomers.

Replace `fabric serve` with `grunt serve`.
Replace `fabric build` with `grunt build`. This also replaces the `<script data-main>` automatically.

:warning: You'll need to run npm install after pulling.

Fix #38
Merge pull request #87 from luqmaan/38-fix-deployment

Add Contributing instrictions to README, Setup grunt