add porting guide ref to 1.2.0 item

doc/html/changelog.html

@@ -67,7 +67,7 @@
 			<li>
 				FLAC format:
 				<ul>
-					<li>(none)</li>
+					<li>One of the reserved bits in the FLAC frame header has been assigned for future use; make sure to refer to the <a href="http://flac.sourceforge.net/api/group__porting__1__1__4__to__1__2__0.html">porting guide</a> if you parse FLAC streams manually.</li>
 				</ul>
 			</li>
 			<li>