Add a HACKING document for new code contributors
Originally created by @anonym on #12164 (Redmine)
Recently I’ve found myself repeatedly explaining primarily Tails source tree to new prospective contributors. I think we should have a short document explaining the basics of it (which will be enough for 95% of new contributions) that tries to be self-contained instead of referring to our copious amounts of contributor docs which might scare people away. It should be possible to read in at most 20 minutes or so.
Feature Branch: doc/HACKING