Fix up some README stuff
Fix up some README stuff

- Windows 'snapshots' are dead
- Show how to install from git
- Indicate the need for `PYTHONPATH` if not installing

Change-Id: I5831299f57271d18225d513dfd31f9707c8e714e
Reviewed-on: http://review.couchbase.org/45388
Tested-by: Mark Nunberg <e7d42768707bf23038325b0b68d4b577e5f6064a@haskalah.org>
Reviewed-by: Volker Mische <fb414f8ac0dbbf87663550ae4ef5fc95b1041941@gmail.com>

mnunberg/couchbase-python-client,mnunberg/couchbase-python-client,couchbase/couchbase-python-client,couchbase/couchbase-python-client