Add section on collecting data under development

I lost some time and feel like this was buried too far down in the docs:

    "There’s generally no need to collect metric and experiment data outside
    production environment. Under Rails, Vanity turns collection on only if
    the environment name is “production”. You can control this from
    config/environments by setting Vanity.playground.collecting to
    true/false. When collection is off, Vanity doesn’t connect to the
    database server, so there’s no need to set a database configuration for
    these environments."
     - http://vanity.labnotes.org/rails.html
    
And wound up having to use these resources to figure out exactly what to do:
    https://groups.google.com/d/msg/vanity-talk/RhqxlqyWN-4/FIbXzeKyooYJ
    https://groups.google.com/forum/#!topic/vanity-talk/opVRJN0KDgk