doc: update contribution guidelines

[edkerk]

Description of the issue:

Leveraging on the work done for yeast-GEM, the contribution guidelines should be updated, to clarify the following:

• when major, minor and patch versions are defined
• how to exactly make a new release
• ...

Question: should guidelines be stored in the repo root, in the .github folder or in the repo's Wiki?

[BenjaSanchez]

note that they can also be stored in /docs, but as you already have a folder named /doc it's probably not a good idea

[simas232]

I suggest to put the contribution guidelines in .github folder. In RAVEN Wiki we can add the reference to that document, no need to add it there as well.