Prepare for 0.91 release.

This includes a note in README that covers the RPATH/ldconfig issue
found #37, and also a note for the parallel installation discussion in
issue #25.
Add an internal script for generating HTML docs.

This script isn't intended for distribution, but I wanted to put it
somewhere it wouldn't get lost. It can be run after the manual pages
have been built to generate HTML versions suitable for putting on the
website.