Automatic "generation" of README from README.md

Instead of manually keeping two versions of 'README' in sync,
the plain text 'README' is "generated" by simply copying the
markdown version to a new name.

The distribution contains only the "plain text" version, not the
markdown version. However, building from a fresh checkout will
warn about "README" not existing, just before creating it :-P
Raised required perl version to 5.8

Some features, notably the 'utf8' pragma and the 'utf8::upgrade'
calls, seem to require perl 5.8 (says 'perlver'). Smoketesting
with 5.6 also failed. Requiring 5.8 is fine by me.