Rework README

[solardiz]

Our current README dates back to when this was only a PAM module. Now we also have a library and tools. We should replace the contents of README to describe the whole project as it currently is, probably moving most of what's currently in README to a new file - e.g., call it OPTIONS?

[ldv-alt]

Yes, this makes sense, there should be a file describing the whole project, but I'm not volunteering to do this job. :)
Given that options are also described in passwdqc.conf(5) manual page, maybe the file describing pam_passwdqc should rather focus on other aspects, e.g. pam stack integration.

[solardiz]

I was thinking that we wouldn't have a separate text "file describing pam_passwdqc" - rather, the options list moved from README to e.g. OPTIONS would describe the configuration options most of which are not specific to the PAM module alone, but are also supported across other components. Yes, it would be duplicate with passwdqc.conf(5) - just different format.

Currently, PAM stack integration would fit (albeit not perfectly) into PAM-using systems' sections in PLATFORMS.