versioning/releasing docs

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers:

********* DESCRIPTION **************
Branch Name: eliu_gh_338_define_and_doc_process_for_tagging_releases
Ticket(s)/Issue(s): Closes #338 

added some docs around how versioning & releasing will work for us

Note: need to bump docs. @sc932 can you do this alongside your changes?

Note: section ```Updating DockerHub``` is incomplete, marked ```TODO(sclark)```. Please tell me what to put here (or check out this branch and add it)

********* TESTING DONE *************
visually inspected docs