Adds sanitised stackforge source repo requirement to docs

Stackforge creation sometimes includes source repos with crufty
branches. Cleaning these branches in stackforge requires -infra
core intervention. Sanitising them prior to seeding the stackforge
repo is easier than cleaning them afterward.

Change-Id: I0d71c9e252913dc781632988df95cc79c626d2b4
Merge "Adds sanitised stackforge source repo requirement to docs"