Document minimum log rounds

[pdilyard]

I had added this line in my test config:

config :comeonin, :bcrypt_log_rounds, 1

which did not make my tests any faster. I looked into the source code, and found that if the value is not between 4 and 32, it gets thrown out and 12 gets used (changing my config to 4 made my tests fast again). I asked about this in the Elixir Slack channel and found that this has confused others in the past.

I think this should either be strongly documented or throw a warning instead of just silently using 12.

Thanks!

[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.