README: fix typo in version numbers

The Fortran ABI break was between 1.8.1 and 1.8.2 (not .2 and .3).
NEWS: Updates for 1.8.4 release.
VERSION: Update shared library version numbers for v1.8.4

(Re-)Added "# JMS" comments to explain why each version number
changed.

NOTE: Some version numbers are annotated with a "# JMS" line, but
didn't change in this commit.  This is because the "# JMS" lines
reflect changes *since the v1.8.3 git tag* -- and some version numbers
had already been changed here in VERSION (in the 1.8.4rc1 timeframe or
so).

The "# JMS" comments will be removed before final release.