Update website with macOS 10.15 issue for SM 2.49.5 r=me

src/releases/seamonkey2.49.5/index.en.html

@@ -171,6 +171,10 @@ Please read this before reporting any new bugs.
     For newer distributions you probably need the Linux x64 version.
   </li>
 
+  <li>SeaMonkey will crash on startup with macOS 10.15. The issue is tracked in
+    <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1584812">bug 1584812</a>
+  </li>
+
   <li>Previous versions of Language Packs should be removed or disabled
     otherwise xml errors will be shown instead of parts of the application.
   </li>