Document PHP requirements like mcrypt

Was looking around for existing encryption modules as I worked out one for Ubercart and came across yours. I checked through the source code to see how you were handling key storage and noticed at the time you're using mcrypt functions. It might be helpful (esp. for folks on shared hosts) to mention on the project page and in the readme that the module requires the mcrypt library to be installed.

(Unless I'm seriously mistaken. }: )