[INTERNAL] Update contribution guidelines to reflect use of git-flow branching model
[INTERNAL] Update contribution guidelines to reflect use of git-flow branching model

notmessenger/sl-ember-modelize,softlayer/sl-ember-modelize,notmessenger/sl-ember-modelize,softlayer/sl-ember-modelize,alxyuu/sl-ember-modelize,alxyuu/sl-ember-modelize