Readme changes - suggestions about HMAC_KEYS and virtualenv

Instead of setting `mozindia` as the virtualenv dir, it is better to recommend `venv` as the virtualenv dir. Added `venv` folder to gitignore to that it is not tracked by git. Also, `HMAC_KEYS` need to be set in `mozindia/settings/local.py`, without which `./manage.py syncdb` will complain about improper configuration.