add branch naming conventions rule

[bgeck]

Add a branch naming convention rule to encourage organization and consistency in feature branch names.

I've found this very useful working in teams, both to help teammates stay organized when thinking about how many chunks to break a TODO into like:

[type]/[task-id]/descriptive-text

feat/1234/datatable-ui-layer
feat/1234/datatable-api-logic
etc....

and also to help communicate to each other and to PMs what a branch is for, even weeks later ;)