Add repo2docker PR tips to contribution guide here #859
Description
repo2docker has some really nice contributing guide, including really nice tips for getting a PR merged. Almost all of these suggestions are general, and probably belong in our top-level org contribution guide, rather than in just one repo's docs.
I suggest we mostly move the repo2docker page to the guide here, and then it can be linked from CONTRIBUTING.md (for most repos), or the more detailed contributing docs for the few repos that have their own contributing details (mainly the big and/or complex ones).