Add a new docs section to explain models evolution compatibility

[rafaparadela]

It would be helpful to explain how models can evolve, and how the serializers preserve back/forward compatibility.

As part of this new section, we can add these conclusions for the Avro cases, but we probably should do the same thing for Protocol Buffer definitions.

[fedefernandez]

@juanpedromoreno this is done, right?

[juanpedromoreno]

Yes, it's partially done since we might want to add the proto section.