README: Document the purpose of this plugin

[korydraughn]

[alanking]

Possible starting words from original white paper (https://irods.org/uploads/2022/Wolfsheimer-Cacciari-SURF-Programmable_authentication_workflows_in_iRODS-paper.pdf):

iRODS (Integrated Rule-Oriented Data System) [1] supports various authentication methods such as native authen-
tication (username and password), GSI, Kerberos, and OpenID. New authentication methods are implemented as
shared libraries that need to be installed on client and server sides. Client libraries such as python-irodsclient may
need to be patched to support any new authentication protocol.

A universal implementation that supports all authentication flows is clearly favored over managing combinations
of client and server libraries and flows. The PAM (Pluggable Authentication Module) [2] mechanism is a way
to implement and customize authentication flows on the server without needing to adjust the software that uses
this mechanism. Existing PAM libraries may be combined to implement flows featuring branches, multiple-factor
authentication, and much more. The PAM mechanism is already supported by iRODS but the current version of
the plugin is restricted to the standard flow only (username and password). We have implemented an authentication
plugin for iRODS 4.3.0 ”pam_interactive” that enables the flexibility of fully-fledged PAM authentication flows
beyond the standard case.

See also...

Talk: https://www.youtube.com/watch?v=wEKQgpPnGk8
Slides: https://irods.org/uploads/2022/Wolfsheimer-Cacciari-SURF-Programmable_authentication_workflows_in_iRODS-slides.pdf