Social Architecture Report: Extensibility

Hello,

I start collection information on how we approach *extensibility*
directly in a wiki page with draft of Social Architecture Report.
https://www.w3.org/wiki/SocialIG/Architecture_TF/ArchitectureFirstWorkingDraft#Extensibility

I find this aspect very important while we put last touch on Social
Syntax and clarify our direction with Social API.

Please feel warmly invited to contribute!

Cheers :)