Update CONTRIBUTING.md with more Contributing Guidelines

[AdamBrousseau]

I think it would be a good idea to add some more contributing guidelines to the CONTRIBUTING.md.
Similar to what what done at eclipse-omr/omr#124.
It would be nice if we could just copy/paste the relevant sections we want from OMR's 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?

[sxa]

There have been various updates to the CONTRIBUTING.md since then, but it would be good to do a check to see if there's anything we could use from the OMR one, so I'm not going to close this, but bump it into the current iteration to get some attention and hopefully work to close it off.

[Willsparker]

Having a brief look at this whilst I go through issues. I've compared the two and these are my thoughts:

• The OMR one links to a Git Crash course file, which, imo, is a very good idea. Could we link to the same crash course in our repo? Or possibly link @sxa 's one?
• 's/AdoptOpenJDK/Adoptium/g' in our one.
• There's some additional points under submitting a contribution in the OMR one, that could probably be added to our one: points 1 and 3 in particular.
• The OMR one has a section that explains issue labelling. While we have issue templates, a small section to explain both those templates and the labels may be helpful.
• Our 'Eclipse Contributor Agreement' section could be bulked out a bit, like OMR's 'Legal Considerations' section, where it explains exactly how to sign the Eclipse Contributor Agreement, as opposed to ours that just links to the agreement itself.

Not too much? I'm happy to make those changes, or everyones in agreement with the above points, or have any additional points they want to add :-)

[smlambert]

For ECA, it may be helpful to link to this doc: https://github.com/adoptium/documentation/blob/main/documentation-documentation/eca-sign-off.adoc