Update rbt to unstable version and update docs.

This change updates rbt for automatically-generated commit message
(./rbt patch --commit) support. It also updates the contributor
documentation to suggest its use for all commits to master.

Testing Done:
This review was posted using the new workflow.

Verified that ./rbt works in a clean repo and one with an old version of
rbtools bootstrapped.

Reviewed at https://reviews.apache.org/r/16073/