Windows: 1.7.23 Release Notes

Add explanation of Win 7 Explorer Shell Read-Only Volume vs Network Drive
Letter Mapping bug.

Change-Id: I7cf1300c9ec91cbbd347030d0bb9642c78607398
Reviewed-on: http://gerrit.openafs.org/9634
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>