Re-added Subversion dependency

- at time svn is used in g.extension to fetch code selectively from grass-addons (on GitHub)

REQUIREMENTS.html

@@ -210,7 +210,9 @@ newer versions are named "python-pillow"
 <li><b>git</b> (git for g.extension)<br>
 <a href="https://git-scm.com/">https://git-scm.com/</a><br>
 </li>
-
+ 
+<li><b>Subversion</b> (svn in g.extension to fetch code selectively from grass-addons on GitHub)<br>
+<a href="https://subversion.apache.org/">https://subversion.apache.org/</a><br>
 </ul>