Add a release note about package trust

docs/users_guide/7.2.2-notes.xml

@@ -9,6 +9,14 @@
   </para>
 
   <itemizedlist>
+    <listitem>
+      <para>
+        All packages are now trusted by default. If you wish to make use
+        of Safe Haskell, you will need to manually untrust packages that
+        you do not wish to trust.
+      </para>
+    </listitem>
+
     <listitem>
       <para>
         An optimiser bug