Show where attributes are used

[fulldecent]

Description

Adds a note about how attributes work.

Note: Mailchimp is a popular email software. And the way it works is that each list has its own attributes.

I learned that phplist uses the same attributes for all lists, this adding this note in case it will be helpful to other people.

Related Issue

Screenshots (if appropriate):

[michield]

Well, that can be debated. Using subscribe pages, you can choose which attributes are displayed and used for each list.

So, yes, attributes are related to subscribers, and control their profile, but lists map to subscribe pages which allow for a subset of the system wide attributes to be used.