The Usergrid project has adopted a policy for how code is to be contributed by external contributors, and by those who are committers on the project. You can read this policy here Usergrid Contribution Workflow.
Basically, we use GitHub as our code review system. So you should fork the apache/usergrid repo and submit PRs back to the project. Here are are step-by-step guide for both both external contributors and committers:
Documentation is in the /docs directory of our Git repo, written in Markdown format and managed by the Sphinx documentation system. See the README.md there for more information.