openocd.texi: document Tcl RPC and add an example client

openocd.texi: document Tcl RPC and add an example client

This should provide enough information to start using OpenOCD RPC.

I've seen some other example clients in different languages but I
can't find them anymore, and their legal status was unclear.

Change-Id: I3a95fe361d773040d1e52a62f9cc0cc655019a9f
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1915
Tested-by: jenkins
Reviewed-by: Andreas Ortmann <ortmann@finf.uni-hannover.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>