Add url to gdb quick tutorial

doc/tutorial/extending_theano_c.txt

@@ -1000,6 +1000,8 @@ process (e.g. theano-nose):
 $gdb python
 (gdb)r bin/theano-nose theano/
 
+`Quick guide to GDB <https://www.cs.cmu.edu/~gilpin/tutorial/>`_.
+
 Final Note
 ==========