Add contributing guidelines in the README

[bolinocroustibat]

Would protect/help the maintainers against future contributions to have contributing guidelines section ready in the README.
We could use something like this for example.

[bolinocroustibat]

Can make a PR if you want but you might have your team guidelines which I'm not aware of!

[skelz0r]

That's a great idea. I'm going to bootstrap one and ask our team if it suits them.