Added .bazelversion to be consitent with the TF build process (tensorflow#340)

Author: JimClarke5

CONTRIBUTING.md

@@ -44,6 +44,11 @@ configuration script (`./configure`), and copy the resulting
 `.tf_configure.bazelrc` to `tensorflow-core-api`. This overrides the default options, and you can add to it manually (i.e. adding `build --copt="-g"`
 to build with debugging info).
 
+The `tensorflow-core/tensorflow-core-api/.bazelversion` file must be kept in sync with `@org_tensorflow/.bazel_version`. 
+This allows using [Bazelisk](https://github.com/bazelbuild/bazelisk) which runs the bazel version given in .bazelversion instead of having to 
+physically reinstall a specific `bazel` version each time the TensorFlow version changes.
+
+
 ### GPU Support
 
 Currently, due to build time constraints, the GPU binaries only support compute capacities 3.5 and 7.0.  
@@ -127,6 +132,7 @@ bazel-out/k8-opt/bin/external/org_tensorflow/tensorflow/libtensorflow_cc.so --ou
 
 (called in `tensorflow-core-api`).
 
+
 ## Adding Gradients
 
 In some cases, a op supported by Tensorflow Java will not have a gradient defined, resulting in errors like this:

tensorflow-core/tensorflow-core-api/.bazelversion

@@ -0,0 +1 @@
+3.7.2