New guide section on Git and GitHub

[Andrew Moore]

My impression from the mailing list is that going forward Trac would be used exclusively for bug reports.  Any code submissions would ideally be submitted as pull requests to GitHub.  If so, this change needs to put front and center, in large display type.

Section 7 “MacPorts Projects” should begin:  SUBMIT CODE AS GITHUB PULL. REQUESTS.  IF YOU JUST WANT TO $#!@, PLEASE USE TRAC
-AM

> On Apr 11, 2018, at 5:31 PM, Perry E. Metzger <pmetzger at macports.org> wrote:
> 
> I just committed a new section of the guide called "Using Git
> and GitHub". Please read, comment, and improve.
> 
> Perry
> -- 
> Perry E. Metzger		pmetzger at macports.org