Add a note mentioning --view-sunit-dags improvements.

llvm-svn: 52008

llvm/docs/ReleaseNotes.html

@@ -202,6 +202,8 @@ faster:</p>
     non-power-of-2 integer sizes.</li>
 <li>Support for 128-bit integers on X86-64.</li>
 <li>Several compile time speedups for code with large basic blocks.</li>
+<li>Several improvements which make llc's <tt>--view-sunit-dags</tt>
+    visualization of scheduling dependency graphs easier to understand.</li>
 </ul>
 
 </div>