Add Test section in README.md

:panda_face: 
I think that executing the command from the project root is easier to the person understand the current place and directory hierarchy he is placed in, like [in the comment above](https://github.com/Respect/Validation/pull/199/files#discussion_r15655368).

Also, the `phpunit.xml` configuration is not necessary; I would omit it from the example.