Github PR suggestion

[Leif Hedstrom]

Hi all,

since we now have a process which lets us create Github PRs, without first 
filing a bug report / issue, I’ve come to find that it’s many times difficult 
to know what “issue” a particular PR is addressing. So, I would like to suggest 
that we all make an effort to put more Description text in Github PRs, which do 
not have a Github Issue filed already.

I’m also writing up a little bit of this into the CONTRIBUTING.md, see

        https://github.com/apache/trafficserver/pull/1447

Thanks,

— Leif