Update the installation documentation to include the Bazel version.
This will be useful information for anyone who already has Bazel installed.
@@ -77,6 +77,8 @@ sudo pip3 install --upgrade virtualenv # system-wide install
[Install Bazel](https://docs.bazel.build/versions/master/install.html), the
build tool used to compile Tensorflow Federated.
+NOTE: Bazel version `0.19.2` or greater is required by TensorFlow Federated.
+
#### 3. Clone the Tensorflow Federated repository
```bash
