Add an "incompatibility" entry to 9.1 release notes about CREATE EXTENSION.

We've now seen more than one gripe from somebody who didn't get the memo
about how to install contrib modules in 9.1.  Try to make it a little more
prominent that you aren't supposed to call the scripts directly anymore.

doc/src/sgml/release-9.1.sgml

@@ -367,6 +367,24 @@
 
    </sect3>
 
+   <sect3>
+    <title>Contrib</title>
+
+    <itemizedlist>
+
+     <listitem>
+      <para>
+       All contrib modules are now installed with <link
+       linkend="SQL-CREATEEXTENSION"><command>CREATE EXTENSION</></link>
+       rather than by manually invoking their SQL scripts
+       (Dimitri Fontaine, Tom Lane)
+      </para>
+     </listitem>
+
+    </itemizedlist>
+
+   </sect3>
+
    <sect3>
     <title>Other Incompatibilities</title>