ut/user_guide: Clarify requirements for contributors

Created by: nicola-mazzucato-arm

A specific section is introduced to explain better what the expectations are from contributors with regards to unit testing.

In particular, it's worth noting that we are still in an introductory phase and, although we encourage contributors to play & add with UT, we feel that it's fair to give everybody some time to get familiar with the tool and the way it's used within the project.