Add simple documentation for PORO decorators

This adds just two small sentences introducing the possibility of PORO decorators
and links to the github issue where there is a full discussion with lots of context
and the gem 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.
Merge pull request #3688 from carnesmedia/document-poro-decorators

Add simple documentation for PORO decorators