Improve help text for referral rule

[robmoorman]

Instructions how to add a referral rule is too techy, we should add some example or help_text of regex values.

[tomdyson]

As a small improvement, I suggest "regular expression to match the referrer".

As an example, we could use (facebook|google)\.com

As an alternative, we could provide a simple 'referrer' rule. Since we allow repeated rules, users can add facebook.com, google.com etc.

[larswisselink]

Something like; separate referrals by comma, for example: google.com, facebook.com, twitter.com