Update Contributor's Guide with info that VSTS builds for macOS

VSTS is now running unit tests for Python 3.6.5 on macOS
- In the future we will be able to test on multiple versions of Python on macOS using VSTS, but there is currently a limitation to testing with Python installed from Homebrew

Author: Todd Leonhardt

CONTRIBUTING.md

@@ -398,7 +398,7 @@ how to do it.
 
 7.  Creating the PR causes our continuous integration (CI) systems to automatically run all of the
     unit tests on all supported OSes and all supported versions of Python.  You should watch your PR
-    to make sure that all unit tests pass on Both TravisCI (Linux) and AppVeyor (Windows).
+    to make sure that all unit tests pass on TravisCI (Linux), AppVeyor (Windows), and VSTS (macOS).
 
 8.  If any unit tests fail, you should look at the details and fix the failures.  You can then push
     the fix to the same branch in your fork and the PR will automatically get updated and the CI system