ReleaseNotes.html: add note about specifying TLS models

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160168 91177308-0d34-0410-b5e6-96231b3b80d8

Author: zmodem

docs/ReleaseNotes.html

@@ -449,6 +449,8 @@ <h3>
    expose new optimization opportunities:</p>
 
 <ul>
+  <li>Thread local variables may have a specified TLS model. See the
+  <a href="LangRef.html#globalvars">Language Reference Manual</a>.</li>
   <li>...</li>
 </ul>