Update README with Google Test version 1.8.1

Author: ladislas

UNITTESTS/README.md

@@ -85,6 +85,8 @@ Mbed CLI supports unit tests through the `mbed test --unittests` command. For in
 
 A unit tests suite consists of one or more test cases. The test cases should cover all the functions in a class under test. All the external dependencies are stubbed including the other classes in the same module. Avoid stubbing header files. Finally, analyze code coverage to ensure all code is tested, and no dead code is found.
 
+Unit tests are written using [Google Test v1.8.1](https://github.com/google/googletest/releases/tag/release-1.8.1).
+
 Please see the [documentation for Google Test](https://github.com/google/googletest/blob/master/googletest/docs/primer.md) to learn how to write unit tests using its framework. See the [documentation for Google Mock](https://github.com/google/googletest/blob/master/googlemock/docs/Documentation.md) if you want to write and use C++ mock classes instead of stubs.
 
 #### Test suite configuration