Document minimum log rounds about comeonin

[riverrun]

Yes, it's documented in the gen_salt function, but not in the hashpwsalt function, which might be confusing. Maybe raising an error would provide more forceful 'documentation' :)
I'll let you know what I decide on.

[pdilyard]

At the very least I'd recommend adding a little note to the README in step 4 of the Installation section.

[riverrun]

I've just released version 2.4, and these are the changes:

• If the log_rounds is less than 4, then gen_salt will use 4 log_rounds, and if log_rounds is more than 31, gen_salt will use 31 log_rounds (this is the default behavior for the OpenBSD version of bcrypt). If the log_rounds argument is the wrong type, then an error will be raised.
• Added documentation to bcrypt_log_rounds in the Config module.

[pdilyard]

Awesome, thanks! I think this will help keep a lot of people from tripping up.