oneMKL Unit test documentation

[rodburns]

The description of how to run the unit tests assumes knowledge of Google Tests framework. Document how to run the unit tests in the CONTRIBUTOR page or link to the instructions. Include information on running tests on multiple targets.

[vbm23]

@mkrainiuk Would it be possible to include details in oneMKL Contribution Guidelines which enlist the steps and commands to run tests on all expected target platforms as GoogleTest is already mentioned under Unit Tests, not sure if you want for "Performance Testing" as well.

You can refer #58 if you like to.