[wwwdocs] Mention -Wshift-overflow

[Marek Polacek]

I've committed this patch to document recent addition.

Index: changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/changes.html,v
retrieving revision 1.14
diff -u -r1.14 changes.html
--- changes.html	14 Jul 2015 12:55:08 -0000	1.14
+++ changes.html	21 Jul 2015 09:29:51 -0000
@@ -43,6 +43,10 @@
     <li>A new command-line option <code>-Wshift-negative-value</code> has been
 	added for the C and C++ compilers, which warns about left shifting
 	a negative value.</li>
+    <li>A new command-line option <code>-Wshift-overflow</code> has been
+	added for the C and C++ compilers, which warns about left shift
+	overflows.  <code>-Wshift-overflow=2</code> also warns about
+	left-shifting 1 into the sign bit.</li>
   </ul>
 
 <h3 id="c">C</h3>

	Marek