Notes on contributing

[Julio Merino]

Hi all,

Now that some of you are starting to provide pull requests (thanks
much!), I have written a bunch of guidelines on how to properly format
patches and history.

Following these will avoid surprises and unnecessary turnarounds for
trivial details:

https://github.com/jmmv/kyua/wiki/Contributing

If you could take a look and consider these in your changes, I'd be
much appreciated.

Thank you!