Document how to run subsets of tests

Full test suite takes A LOT of time due to running hash on spark multiple times.
We have tests that don't depend on hashing or external services they are [tagged correctly](https://github.com/src-d/gemini/tree/master/src/test/java/tags).
Mention them in readme for developers with a command that one can just copy-past would help during development.