Add some prose describing what is the reader required to implement on each exercise.

[vic]

thanks to all to have contributed content and exercises to rodeo. one thing I believe we can improve is, adding more descriptions about what should I do on each exercise, I've seen we have exercises that are only code (some preliminar code and asserts) but it would be awesome to also have some prose on what am I expected to do, perhaps a description of what the asserts are testing.