http://drupal.org/node/11517. I have done this using some command line magic:

find . -name README -exec cp '{}' '{}.txt' \; -exec cvs remove -f '{}' \; -exec cvs add '{}.txt' \;

I will update all occurences of 'README' to 'README.txt' in the conrtib CVS,
soon. I'll also rename LICENSE, CHANGELOG etc. files as soon as I get around
to it.