Add node-and-timestamp to README.rst

Previously only `node-and-date` and `dirty-tag` were listed as available implementations.

Author: sTywin

README.rst

@@ -272,6 +272,8 @@ Version number construction
 
     :node-and-date: adds the node on dev versions and the date on dirty
                     workdir (default)
+    :node-and-timestamp: like :code: `node-and-date` but with a timestamp of
+                         the form :code: `{:%Y%m%d%H%M%S}` instead
     :dirty-tag: adds :code:`+dirty` if the current workdir has changes