Add require 'pdf_forms' to readme 

Hello,

This is a wonderful library and thank you so much for creating it!  I have spent several days researching a simple way to fill in pdf forms and everyone on the internet seems to think prawn is the best tool for that (it is not).

Anyway, I think you should add this explicit require to the readme since the gem name and the require name differ. It took me a few minutes to figure this out and I eventually had to look it up in the test_helper.  A minor nitpick, but I think it will help out new people trying to use the gem for the first time.

thanks again