ACTION-217: updatd boilerplate on optional algorithms

I've updated the introduction of the algorithms cross-reference note  
to indicate that algorithms should be assumed optional when this note  
doesn't say anything about their support in other specs.

http://www.w3.org/2008/xmlsec/Drafts/xmlsec-algorithms/

Thanks,
--
Thomas Roessler, W3C  <tlr@w3.org>