Add simple documentation for PORO decorators

This adds just two small sentences introducing the possibility of PORO decorators
and links to the [github issue](https://github.com/activeadmin/activeadmin/issues/3600) where there is a full discussion with lots of context
and [the gem](https://github.com/kiote/activeadmin-poro-decorator) by @kiote which not only is a solution but also serves as documentation
for what is required to write your own PORO decorators for ActiveAdmin.