README: Update WhatsApp section.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
!sms: Added (undocumented) support for sending to WhatsApp groups.
      In user prefs, set phone to "g" then the group JID, i.e. !pref phone=g1234567890-1234567890
      For help finding a group's JID, try https://github.com/tgalal/yowsup/issues/162

Signed-off-by: Martin Stone <martin@d7415.co.uk>
README: Remove old method for WhatsApp password as it's unlikely to work for anyone anymore.

Signed-off-by: Martin Stone <martin@d7415.co.uk>