t/porting/readme.t: Check Porting/README.pod consistency

This is a slightly expanded (or bloated) version of the patch
Dennis Kaarsemaker <dennis@kaarsemaker.net> submitted.
Fix Porting/README.pod so it passes its new test.

Added entries for sync-with-cpan and README.pod itself.  Rearranged
entries to be sorted consistently.
MANIFEST: add t/porting/readme.t