Merge pull request #257 from kkrawczyk123/readme_update

Update README.md file

Author: yanokwa

README.md

@@ -57,6 +57,7 @@ If you're ready to contribute code, see [the contribution guide](CONTRIBUTING.md
 All releases are verified on the following operating systems:
 * Ubuntu 17.04
 * Windows 7
+* OS X 10.11.6
 
 Testing checklists can be found on the [Briefcase testing plan](https://docs.google.com/spreadsheets/d/1H46G7OW21rk5skSyjpEx3dCZVv5Ly4WDK8LISmrz714/edit?usp=sharing).