Update contributor guidelines

[zregvart]

We should update the contributor guidelines to talk about:

• conventional commits (more details, examples)
• good commit messages (having title and body explaining the change)
• pull request sizes (doing work behind feature toggles, incrementally)

[lburgazzoli]

Should we include some info about formatting and testing rules ?

[kahboom]

+1 @lburgazzoli - That could be useful.

And we should probably remove the stuff about ZenHub since it's outdated, and update the development workflow.