CONTRIBUTING.md include basic Docker CI command

Author: ahundt

CONTRIBUTING.md

@@ -159,8 +159,11 @@ There are two ways to run TensorFlow unit tests.
    bazel test ${flags} //tensorflow/python/...
    ```
 
-2. Using Docker and TensorFlow's CI scripts.
-
+2. Using [Docker](www.docker.com) and TensorFlow's CI scripts.
+   ```bash
+   # Install Docker first, then this will build and run cpu tests
+   tensorflow/tools/ci_build/ci_build.sh CPU bazel test //tensorflow/...
+   ```
    See
    [TensorFlow Builds](https://github.com/tensorflow/tensorflow/tree/master/tensorflow/tools/ci_build) for details.