Documentation change: adding a note about security rules and linking to #432 (#438)
Documentation change: adding a note about security rules and linking to #432 (#438)

* Adding a note about security rules and linking to #432 
* Fixing the markdown link
* Addressing @tstirrat's review: fixing language and clarifying
FirebaseExtended/emberfire,FirebaseExtended/emberfire,FirebaseExtended/emberfire,FirebaseExtended/emberfire