Installation instructions

Hi Bruce. I'd like to propose that we use the sgml sources for the
installation instructions. That will give us one set of instructions
which can appear in postscript, html, and plain text. I'll generate a
plain text version which we can put into the top-level directory as
usual.

The plain-text version can include the accurate porting info and release
notes also, since those are in sgml.

I notice that INSTALL was updated on Aug. 30, but that the sgml version
of the same instructions (install.sgml) was not, so we would have to
remerge again anyway...

                      - Tom