Update contributing documentation

[david4096]

I believe this affects multiple repos so starting here.

From @dcolligan's comment here

Here's how I would advise doing it:
- push a branch to ga4gh/schemas and open a PR on it
- open a PR with your contstraints.txt file line for schemas pointing to the remote schemas branch
- presumably both the schemas and the server changes are going in, and the server depends on schemas, so merge the schemas PR first, once it is approved
- change your server PR to use the default version of constraints.txt (that is, the one that points to all the master branches)
- merge the server PR

These docs need to be reflected in our contributing documentation. Now that we've figured out how to do multiple repository release, it should be a lot easier to understand how to push the model forward. That being said, it's kind of hard to know which repo to issue a PR at when first coming to the stack.

FAQ?