warning for broken feature copying external sources

manual/tutorial_external_sources.html

@@ -72,7 +72,10 @@ <h3>Referring to source descriptions in other files</h3>
           file must be capable of handling this reference in order to retrieve
           all source information.</li>
       </ul>
-      <h3>Copying source descriptions from another file</h3>
+      <h3>Copying source descriptions from another file</h3>      
+      <ul class="important">
+        <li>Warning: This feature is currently broken, see <a href="https://github.com/Edirom/MerMEId/issues/194">GitHub issue #194</a></li>
+      </ul>
       <p>Instead of referring to a source description contained in another file, you may want to copy 
         the external source description into the current file. This is convenient if you need to change something in the 
         source description in order to adapt it to the current file. </p>