Document crypto_provider in README, and help w/SCrypt confusion

This is a solution to solve the confusion from the scrypt change in 3.4.0:

https://github.com/binarylogic/authlogic/pull/417
https://github.com/binarylogic/authlogic/issues/411
https://github.com/binarylogic/authlogic/issues/416

It outputs an explanation for the missing scrypt gem for newcomers. 

Other alternative: to actually require scrypt in the gemspec?