Add warning about .stand-off.vrt

web/app/templates/corpora/add_corpus_file.html.j2

@@ -3,7 +3,13 @@
 {% block page_content %}
 <div class="col s12 m4">
   <h3>{{ corpus.title }}</h3>
-  <p>Fill out the following form to add a corpus file in verticalized text format (.vrt).</p>
+  <p>
+    Fill out the following form to add a corpus file in verticalized text
+    format (.vrt).
+  </p>
+  <p>
+    <strong>Do not use the .stand-off.vrt file!</strong>
+  </p>
   <a class="waves-effect waves-light btn" href="{{ url_for('corpora.corpus', corpus_id=corpus.id) }}"><i class="material-icons left">arrow_back</i>Back to corpus</a>
 </div>