Update r178 release notes (Add "Known issues")

Author: ychin

_includes/releases/r178.html

@@ -41,6 +41,12 @@ <h1 id="fixes">Fixes</h1>
   <li>Fixed Vim occasionally crashing and/or hung when autocmd calls <code>serverlist()</code> on exit. <a href="https://github.com/macvim-dev/macvim/issues/1427">#1427</a></li>
 </ul>
 
+<h1 id="known-issues">Known issues</h1>
+
+<ul>
+  <li><code>:Man</code> command doesn’t work properly if <code>gdefault</code> option is set. <a href="https://github.com/vim/vim/issues/13097">vim/vim#13097</a>.</li>
+</ul>
+
 <h1 id="scripting">Scripting</h1>
 
 <ul>