doc: README: split out ancillary information

doc: README: split out ancillary information

The README was becoming too long and contained
quite a bit of info only pertaining to rarely used systems, so...

* README: Split out install specific info to README-install.
Also remove a few stale lines, and reorder a few items.
* README-install: A new file split from README.
* Makefile.am [EXTRA_DIST]: Explicitly reference new README-install
file for distribution, since automake only auto adds README.
* TODO: Reference the HPUX info now in README-install.