README: add nuance to description of git clone --depth

Closes #47
Suggested-by: Aleksander Nitecki
Fix syntax and error cleanly on Python-3

Python-3 requires careful handling of unicode to avoid breaking Git
semantics (which manages strings unencoded) or Python (which insists on
processing encoded strings on Windows).  This is not done yet, so error
cleanly for now.  See discussion in issue #42.

Suggested-by: Christoph Buchner