Cachekey update helper: Document ensuring . in PYTHONPATH

The current directory isn't always in the python module search path,
so we have to ensure it is for the script to work.

Strictly speaking, the user may already have a modified PYTHONPATH
at which point PYTHONPATH=".${PYTHONPATH+:$PYTHONPATH}" is necessary,
but it's probably premature to overcomplicate the documentation like that
before we discover it's a problem.