Update CONTRIBUTING.md with more Contributing Guidelines

I think it would be a good idea to add some more contributing guidelines to the [CONTRIBUTING.md](https://github.com/AdoptOpenJDK/openjdk-infrastructure/blob/master/CONTRIBUTING.md).
Similar to what what done at eclipse/omr#124.
It would be nice if we could just copy/paste the relevant sections we want from [OMR's CONTRIBUTING.md](https://github.com/eclipse/omr/blob/master/CONTRIBUTING.md). We would just have to figure out the license/copyright hoops to jump through to do that.

Perhaps @tellison has some insights on the legal aspects?

Start with CONTRIBUTING.md in this repository and compare it with OMR's CONTRIBUTING.md linked in the issue. Identify the relevant sections to copy, then determine the required license and copyright permissions, including the question raised for @tellison. Done means the selected guidelines are added with the legal requirements resolved.

Written by the indexing model from the issue text.