Add how to use as module to the README

[hayd]

Fix #148.

Note: IMO the FormatFile api is a little strange atm, in returning (source, encoding) or (diff, encoding). I think an in_place argument would be good... but I'm not sure what it ought to return in this case (None, encoding) ?

[hayd]

eurgh, this is a typo. Doh!

[bwendling]

Go ahead and fix it and then squash the result. :-)

[bwendling]

Oops! I didn't catch this. I mentioned in the other pull request that it should probably return None, encoding. That seems the most logical thing to do, IMHO. But I could be wrong. :-)

[hayd]

I may cheekily wait for #158 and then update to include mention of in_place! :)

Like I said there, having looked a little, I'm not sure that (None, encoding) makes sense, because then you have the same result whether:

• the file exists, is reformatted, writes out sucessfully
• the file does not exist, and exits*

* for some reason currently this returns (None, encoding) rather than raising an exception of some sort... :s

Edit: The docstring was incorrect, so this isn't actually the case. FormatFile raises if there file is not found.