Add `CONTRIBUTING.md`

[xvw]

The aim is to make it easier for outsiders to contribute. Here is a first version of a contribution document. At first I'd added examples (ie: how to add a custom query, what Fiber is and how to use it etc.) but I've decided to remove them to focus the review on ‘this base’. As I'm not a native English speaker, I'm open to any rewording. (And of course to any modifications, in general).

Thanks in advance.

[xvw]

Thanks for the review @rgrinberg, I try to take it account the last commit! WDYT?

[voodoos]

LGTM, thanks @xvw

[coveralls]

Pull Request Test Coverage Report for Build 4314

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 21.863%

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Totals
Change from base Build 4312:	0.0%
Covered Lines:	5368
Relevant Lines:	24553

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💛 - Coveralls