add advice on using namespaces in diazo themes

[polyester]

a number of Diazo themes have issues with things like pop-up modals and ajax. The cause is usually that css class-names collide, and therefore inadvertently trigger Diazo rules.

We should include in the themer documentation a hint / best-practice to use namespaced classnames in their themes. So, instead of 'body' or 'content' use 'mytheme.body' and 'mytheme.content' to avoid conflicts.