Fix bad CLA URL in CONTRIBUTING.md
Rename help text files to <cmd>.txt

The .txt extension enables customary handling for text files in my
editor, and probably other editors, like wrapping.
pmctl: add timeout argument to cpu-start
pmctl: reorganize long form usage

Move the long paragraph descriptions from the footer to make them inline
with the commands they document. Also, rename W to ID, as it was for
`slc runctl`.