Docs: Reduce contributing guidelines by linking to contribute.jquery.org

This is an initial attempt to reduce our contributing guidelines by offloading the majority of the documentation to our centralized contribution documentation, http://contribute.jquery.org. @jzaefferer and I spent some time this morning fleshing out http://contribute.jquery.org/code/ and the associated pages (currently only on stage). I'd like to do this for our larger projects, which all have pretty lengthy contributing guidelines.