Add PostgreSQL trademark notice

Author: ssinger

doc/adminguide/legal.sgml

@@ -45,6 +45,7 @@ SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
  <trademark>Solaris</trademark> is a registered trademark of Sun Microsystems,
  Inc. <trademark>Linux</trademark> is a trademark of Linus Torvalds. 
  <trademark>AIX</trademark> is a registered trademark of IBM.
+ PostgreSQL is a registered trademark of the PostgreSQL Community Association of Canada.
 </para>
 </legalnotice>