Add contributing guidelines and the 'Bill and Ted' code of conduct to the project

CODE_OF_CONDUCT.md

+There is only one rule here : be excellent to each other, and party
+on.
+
+Until it proves too much of an burden, I, the original contributor,
+will take care of strictly enforcing that rule. I will try my best to
+be fair, but if you still find me lacking in that respect, you are
+welcome to open a "General Issue" about your disagreement (at [this
+address](https://github.com/lih/BHR/issues/new/choose)). 

CONTRIBUTING.md

+So you haven't run away from the front page, and what's more, you even
+want to help make things better ? Thank you ! 
+
+That being said, I haven't the slightest idea what sorts of rules
+would be good to have, or not have, for my hypothetical future
+contributors to feel more welcome.
+
+Hence, until I'm overwhelmed with contributions, there'll only be one
+rule : have fun with it. Write the ugliest code you can, or the most
+elegant, but write something that makes you smile. This project was
+started for fun, and I intend to keep it that way for as long as I
+can.