add pointer to cartman source

tsa/howto/trac.mdwn

 Commandline access
 ==================
 
+We use [cartman](https://github.com/tamentis/cartman/), a "commandline trac client" which "allows you to
+create and manage your Trac tickets from the command-line, without the
+need to setup physical access to the Trac installation/database".
+
 Install:
 
     virtualenv --python=python3 --system-site-packages ~/.virtualenvs/cartman