write plone.app.contenttypes best practices

[svx]

extend currrent docs about plone.app.contenttypes with best practices with reference link to dexterity

[svx]

@pbauer is there already documentation which we can fetch ?

[pbauer]

@svx there only is https://github.com/plone/plone.app.contenttypes/blob/master/docs/README.rst at the moment. I'm not sure what else we might need. Can you give me a tip on some topics?

[svx]

@pbauer looks great, but we should structure that a bit more, atm we have a lot what is great ! But we should improve it, it is one long README :)

[polyester]

@pbauer, I think everything is there. It just needs splitting up. From the styleguide: the README should be a short description of the add-on, not the entire documentation! (Or in this case, the entire unabridged edition of "War and Peace" ;-) When my cold is getting better, I'll fix.

[stevepiercy]

Superseded by https://github.com/plone/documentation/pull/1455