Some doc in README, and a tentative usage of exceptions

The first two commits just add some (IMHO) valuable information in the README file.

The last commit could be a bit more controversial, but it makes things much nicer to work with in higher layers of the stack (pycangjie and ibus-cangjie).

If you don't like the last one, I could work around it in pycangjie anyway, just tell me and I'll drop it from this pull request so you can still merge the first two.

(sorry, I don't know how to split commits in various pull requests with Github :( )