add pull-request etiquette

[escattone]

Fixes mdn/yari#1098

[chrisdavidmills]

Hey @escattone ! Looking mostly good; I've left you a few comments to consider.

[peterbe]

All good stuff! ...except the unusual spelling with the hypen. :)

[escattone]

@chrisdavidmills I think I've addressed all of your points of feedback. Back to you!

[escattone]

Thanks for the review @peterbe! I think I've addressed your concerns as well. Back to you!

[peterbe]

One thing is that people might be confused as to what to put into the pull request description.
If it's solving an open issue just typing "Fixes #1234" is often all you need to say because the issue itself should describe the problem, so the PR implies the solution.

But I can't think of a great thing to suggest there. Perhaps, "If your pull request isn't referring to a specific issue, please try to mention what problem the pull request is solving."

[chrisdavidmills]

@escattone a few more bits, most of them just nits.