Add a particular type of resource to the form.
Forms may contain default resources usable by any of the fields
and annotations in the form. The most common resource types are
"Font", "XObject" and "ColorSpace". For further details see the PDF
This method allows you to add new resources to the form. You may
supply your own name for the resource but if the name is already in
use, it may need to be modified. For this reason the function
returns the value which was actually used for the addition.