Update CONTRIBUTING.md for Travis CI (apple#1210)

Include a mention that we are using Travis CI to test contributions.

Author: znation

CONTRIBUTING.md

@@ -6,3 +6,6 @@ that your contributions are licensed under the [BSD-3 license](LICENSE.md).
 
 Before submitting the pull request, please make sure you have [tested](BUILD.md) your
 changes by running Python and C++ unit tests.
+
+In addition, by submitting the pull request, you acknowledge that the Travis CI tool
+may be used to test changes that are submitted.

README.md

@@ -152,4 +152,6 @@ If you want to build Turi Create from source, see [BUILD.md](BUILD.md).
 Contributing
 ------------
 
-See [CONTRIBUTING.md](CONTRIBUTING.md).
+Prior to contributing, please review [CONTRIBUTING.md](CONTRIBUTING.md) and do
+not provide any contributions unless you agree with the terms and conditions
+set forth in [CONTRIBUTING.md](CONTRIBUTING.md).