README cleanups

[Gary V. Vaughan]

With the recent additions I made to README, I noticed that we're
maintaining a lot of duplicated redundant information, and also that
README and README.alpha have needlessly drifted apart, effectively
hiding useful information from users of our alpha and stable release
tarballs.

All but the last patch in this series consolidate the differences
between these two files to make sure all the useful README details
are shipped in both alpha and stable releases.

At that point, the only difference remaining is the wording of the
very first paragraph - rather than having to make future additions
to both files, I've deleted README.alpha and replaced it with a
dist-time sed script which edits the README file in situ to contain
the alpha wording, along with a change to the Makefile dist-hook
rule to call it.

Tested by inspecting the contents of README in a tarball after
`make dist' with the AC_INIT version number set to 2.2.11a and 2.4.0
respectively.

Okay to push?

Cheers,
-- 
Gary V. Vaughan (address@hidden)