Also link to the github site for compatible pyusb version.

(For various reasons I do not use pip).

Signed-off-by: Chris Lamb <lamby@debian.org>
Document that we need Python 2.7+

At least for collections.OrderedDict(). Debian stable is 2.7, so
doesn't seem worth trying to use some shim.

Signed-off-by: Chris Lamb <lamby@debian.org>
Merge pull request #68 from lamby/python-version

Document that we need Python 2.7+
Merge pull request #67 from lamby/pyusb-link

Also link to the github site for compatible pyusb version.