Charm interfaces docs

[Nick Veitch]

Hi all,
Pursuant to discussions at the sprint on how to make implementing
interfaces easier for charm authors, I have written an initial
document for the mysql charm:
https://docs.google.com/a/canonical.com/document/d/1WTws_k5K__NAsfZwxf-oKHYqZnZYvrg7K58MvOHFpAY/edit#
 It would be very cool if people could add any comments here or to the
doc itself. The thing about publishing reference material is that it
isn't supposed to change! So it would be good to get a general
consensus that this is definitely how the interface works, and the
provided information is useful before we add it to the docs.

Nick