Add rule for branch naming in contributing.md file.
Add rule for branch naming in contributing.md file.

m4rcsch/lithium-core-library,jails/lithium,jails/lithium,UnionOfRAD/lithium,muyehub/lithium,pixelcog/lithium,whowgames/lithium,muyehub/lithium,graphis/lithium,pixelcog/lithium,graphis/lithium,davidpersson/lithium,SensCritique/lithium,wbeater/lithium,SensCritique/lithium,UnionOfRAD/lithium,datachand/lithium,mehlah/lithium,whowgames/lithium,davidpersson/lithium,m4rcsch/lithium-core-library,wbeater/lithium,mehlah/lithium,datachand/lithium