Commit log message guidelines for Development Workflow

[RjOllos]

I'm working on filling in some gaps in the developer documentation, mainly documenting things we already do but haven't written down. The main action items are captured in (1). This will answer some questions that have been raised by contributors and make it easier to onboard new developers.

I'd like to also add some brief commit log message guidelines. Lately I've been following the Git guidelines, which are documented in (2, 3), among other places. I'd like to hear opinions from other committer on adopting the Git commit log guidelines.

Project have slightly different approaches to this, as I've learned (4). My goal is to have some guidelines that give us nice formatting but aren't overly pedantic.

- Ryan

(1) https://trac.edgewall.org/ticket/12358#comment:1

(2) http://chris.beams.io/posts/git-commit/

(3) https://git-scm.com/book/ch5-2.html

(4) https://github.com/python-babel/babel/pull/239#commitcomment-13348577

[RjOllos]

Added in (5). Feedback from contributors is welcome.

- Ryan

(5) https://trac.edgewall.org/wiki/TracDev/DevelopmentWorkflow?version=17