Create contributing guide for schema changes

[k15z]

Notes from @csala:

Oh, and I would also separate the Contributing guide in two parts: I would keep the current one as it is, and then add a second one that expands on the Schema changes. There we should revisit the design principles, add emphasis on the different types of updates that one can suggest (add, remove, change) and put more emphasis on the new functionality vs backwards compatibility balance.