Add documentation about using dynamic content, e.g. forms

Add documentation about using dynamic content, e.g. forms.

Here are some discussion threads, there are more:

Forrest: dynamic or static?
20 September 2003
http://thread.gmane.org/gmane.text.xml.forrest.devel/4055

CForms and Forrest
19 April 2005
http://thread.gmane.org/gmane.text.xml.forrest.devel/12437

[RT] Dynamic Content in Forrest
24 August 2005
http://thread.gmane.org/gmane.text.xml.forrest.devel/14690