Add a saugns(1) man page for installation.

Split README into README.md and doc/README.SAU,
the latter installed under share/doc/saugns/.
Install examples/* to share/examples/saugns,
instead of share/saugns. Add a CONTRIBUTING.md,
which also has a section describing Makefile
targets useful for testing and debugging.

Several possibilities exist for where the
man page should be added/removed. Do something
very simple and hopefully sensible:
 * If "man" exists under the install/uninstall
   prefix, use it.
 * Else, go with "share/man" under the prefix.

Some systems (e.g. OpenBSD) do not use compressed
man pages for official packages, while others do.
Add simple check for this less significant detail:
 * If "/usr/share/man" exists,
   compress man page if and only if
   it contains compressed man page(s).
 * Else, compress man page if and only if
   install location contains compressed man page(s).

Change log:
 * 2019-09-07: Add a man page for installation.
       Split README into README and doc/README.SAU.
       Install examples to share/examples/saugns,
       and doc to share/doc/saugns,
       instead of examples to share/saugns.
 * 2019-09-09: Clean up man page.
       Move saugns.1 file to man/.
       Handle man dir possibilities
       more portably in Makefile.
 * 2019-09-13: Update README.
 * 2019-12-14: Handle whether or not to gzip man page
       upon install (always done before),
       according to whether the install location
       or "/usr/share/man" (takes priority if found)
       contains gzipped man pages.
 * 2019-12-21: Update 'doc/README.SAU' on "@label".
 * 2020-07-23: Update man page.
 * 2020-09-19: Update 'doc/README.SAU' on channel mixing.
 * 2021-01-22: Update man page for AUDIODEV environment.
 * 2021-11-24: Rework 'README' into 'README.md', update man.
 * 2021-11-29: Makefile: add 'make check' script -c testing.
 * 2022-02-23: Add "Contributing" to README.md.
 * 2022-07-28: Update website URLs after move.
 * 2022-08-29: Note doc licensing in & install 'README.md'.
 * 2022-12-29: Update man for new -d, and more.
 * 2024-06-24: Add 'notes/TESTING.md', update 'README.md'.
 * 2024-08-04: Move the 'README.md' 'Contributing' section
      and 'notes/TESTING.md' into 'CONTRIBUTING.md'.
      Add brief text on conduct and source rebasing.