Short documentation on how to use the TestxServer

[b1zzu]

Testx is already working, the first experiment has passed and as it is possible to see from the example it's already working for the most basic use case.

But now we should add some documentation on how to use the TestxServer class. We could also add part of the documentation in the code and then export it using jsdoc in order to keep API documentation close to the code.

Because ours is a small project most of the documentation could just reside in the README.md

Also, we should add a couple of paragraphs explaining the advantages of using graphql-testx over other mocking solutions, part of the motivations are described in these issues: #1 #7