adding some infos to README.markdown about fixtures and the tests

I added the Readme infos about the fixtures and also a line about how to run the test.
I'm not sure what brings `make` in on any OS I'm afraid, if it is gcc then I would assume developers will need this to run the project anyway and would be happy to have that added as a requirement.

However, developer dependencies aside, I think `make run` should be the only API to run the tests that we publish. This avoids the documentation being accidentally outdated if we need to change the test running command.