Added a CONTRIBUTING section to the README

This is based off of https://github.com/phase2/grunt-drupal-tasks/pull/7. Please review/merge that first.

This adds a new top-level section to the README for contribution instructions. I don't have a clear sense that we have coding standards or process on this project to any great extent, so it focuses on explaining how to think about certain types of contributions.
Modifying the existing tasks in your project would be in Customizing, not Contributing :)

That said, you should be able to just use grunt.config.set() to override whatever configuration we've got in the system, the trick is making sure API calls like that come later in the bootstrapping process of all the code. The configuration is all assembled before tasks are run. We should talk about what you are trying to do, maybe there are changes to be made to more naturally support it?