Updating CONTRIBUTING; links and add security notice.
Updating CONTRIBUTING; links and add security notice.

muyehub/lithium,davidpersson/lithium,whowgames/lithium,UnionOfRAD/lithium,datachand/lithium,m4rcsch/lithium-core-library,wbeater/lithium,muyehub/lithium,graphis/lithium,UnionOfRAD/lithium,graphis/lithium,wbeater/lithium,m4rcsch/lithium-core-library,davidpersson/lithium,datachand/lithium,whowgames/lithium