Contribution docs

This change will add the formal Contributor License Agreement (CLA) as well as documenting the Pull Request process for other contributions later.
We should probably rephrase 4 so that it is `Write code and tests` with underlying points from 3, 4 and 5:

1. Write code the follows our [code style guide](CODE_STYLE_GUIDE.markdown)
2. Write corresponding tests
  * Include your tests in the 'test' directory in the appropriate test file
  * Run tests using `grunt test`
3. Ensure automated tests pass

Then point 5 doesn't need any sub-points.