Include persona labeling information towards PM and UX handbooks
Regarding discussions at:
- https://gitlab.slack.com/archives/C0NFPSFA8/p1547214288238500
- https://gitlab.slack.com/archives/C03MSG8B7/p1547214344104300
Adds information on how and where to mention persona's and to label issue's/epic's respectively.
Edited by Dimitrie Hoekstra