Add mention of BenchmarkProbe interface in "Creating Yardstick Benchmarks" section of readme

[ghost]

Suggest to add mention of BenchmarkProbe interface in "Creating Yardstick Benchmarks" section of README.md. Text could be like:

BenchmarkProbe interface is an important building block of the framework - use standard implementations supplied by Yardstick or create your own.

I think that way it would be easier to readers to grok how framework works and what is expected when creating own benchmarks.

Current wording can leave impression that besides implementing BenchmarkServer and BenchmarkDriver interfaces nothing else is needed to integrate with the framework. And even cursory glance on yardstick and ignite-yardstick code may still keep such an impression.