Accepting contributions

First, some housekeeping:

• Make sure that the contributor has signed Google's contributor agreement.
• If it's their first contribution to stagehand, ask them to add themselves to the AUTHORS file.
• If you aren't sure what a template should contain, ask someone who is to review.

Then get the files ready to commit:

• Get the files.
  git pr 123
• Regen, just in case. (Travis does CI, but nothing makes sure that the generated files are the same as the checked-in files.)
  dart tool/grind.dart build
• Update CHANGELOG.md if they didn't already. (Start an ## unreleased section at the top for the first commit after a release. Then change that to an actual number once we know what it'll be.)

Now you can release, or if you're expecting more changes soon, hold off. Instructions for making a release are in Pushing a new version.