Cherrypick fixes made directly to master back into develop (#619)

* Check result of window.confirm before deleting variables.

* Create CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+# Contributing to Blockly
+
+Please make pull requests against develop, not master.  If your patch needs to go into master immediately, include a note in your PR.
+
+For more information, head over to the [Blockly Developers site](https://developers.google.com/blockly/guides/modify/contributing).

core/workspace.js

@@ -322,9 +322,12 @@ Blockly.Workspace.prototype.deleteVariable = function(name) {
           return;
         }
       }
-      window.confirm(
+      var ok = window.confirm(
           Blockly.Msg.DELETE_VARIABLE_CONFIRMATION.replace('%1', uses.length).
           replace('%2', name));
+      if (!ok) {
+        return;
+      }
     }
 
     Blockly.Events.setGroup(true);