Add hoedown COPYRIGHT back

COPYRIGHT

@@ -197,6 +197,28 @@ their own copyright notices and license terms:
     USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
     OF SUCH DAMAGE.
 
+* Hoedown, the markdown parser, under src/rt/hoedown, is
+  licensed as follows.
+
+    Copyright (c) 2008, Natacha Porté
+    Copyright (c) 2011, Vicent Martí
+    Copyright (c) 2013, Devin Torres and the Hoedown authors
+
+    Permission to use, copy, modify, and distribute this
+    software for any purpose with or without fee is hereby
+    granted, provided that the above copyright notice and
+    this permission notice appear in all copies.
+
+    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR
+    DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE
+    INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+    FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+    SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR
+    ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA
+    OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
+    OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+    CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
 * libbacktrace, under src/libbacktrace:
 
     Copyright (C) 2012-2014 Free Software Foundation, Inc.