Add some release notes that say we removed the CellSPU port.

llvm-svn: 168014

llvm/docs/ReleaseNotes.html

@@ -670,6 +670,8 @@ Release Notes</a>.</h1>
    from the previous release.</p>
 
 <ul>
+  <li>The CellSPU port has been removed. It can still be found in older
+      versions.</li>
   <li>...</li>
 </ul>