in [Readme], delay calling ->distmeta until file munging time

Same as in https://github.com/xenoterracide/Dist-Zilla-Plugin-Test-PodSpelling/pull/21 -- if distmeta runs too early, some data can be missing.

https://metacpan.org/pod/Dist::Zilla::Plugin::VerifyPhases now exists to make some of these issues more apparent.