Use gitpod.io for documentation work - round 2

[betatim]

Follow up tasks from #908:

• work out what a "gitpod link" that opens the editor in a documentation file (not README.md) or a file that welcomes people (some instructions) looks like
• mention gitpod as an option for contributing to the docs in CONTRIBUTING
• add a badge to CONTRIBUTING
  • they have a badge like mybinder.org does and I am sure it is somewhere in https://www.gitpod.io/docs/ but I can't find it
• add badge to the README(?)

Anyone can help out with these tasks, some are 10min of work (find link, find badge) and we can post the result here. Or if you have more than 10min make a PR that adds the link into one of the documents.

Labelling this as good first issue because it contains several small tasks.

[choldgraf]

note that somebody opened a PR in the jupyter org docs to do something like this, is that worth looking at and/or adopting here? jupyter/jupyter#525