Update README.md

* Add precisions on the contribution process.
* Add precisions on end to end testing.
Not sure about this line. Sounds a bit as if we'd merge everything that passes the tests. I'd rather go in the direction of "Please make sure that your changes pass the Travis tests before submitting a pull request. We'd also like to ask you to join our IRC channel, #glowing-bear on freenode, so we can discuss your ideas and changes."