doc: note about beta versions of Xcode on OSX

doc/index.html

@@ -695,7 +695,7 @@ USE_OSGPLUGIN(<plugin2>)
 
     <p>
     Install
-    <a href="http://developer.apple.com/TOOLS/xcode/">Xcode</a>
+    <a href="http://developer.apple.com/TOOLS/xcode/">Xcode*</a>
     and
     <a href="http://www.macports.org/">MacPorts</a>,
     then run:
@@ -710,7 +710,9 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     </p>
     <pre>sudo port install gcc_select
 sudo gcc_select gcc42</pre>
-
+    <p>
+    *Beta and Developer Preview versions of Xcode are not supported. 
+    </p>
     <h3 id="requirements-opensuse">openSUSE</h3>
 
     <!-- http://software.opensuse.org/113/en -->