add label to the two-process section

there used to be a whole file which talked about the work-in-progress on
moving toward the two-process ZeroMQ model, and the qtconsole docs
reference it, but after that work was finished, the work-in-progress
docs were removed.

docs/source/overview.txt

@@ -175,6 +175,8 b' Main features of the interactive shell'
   prompts and whitespace) and uses doctest-compatible prompts and
   output, so you can use IPython sessions as doctest code.
 
+.. _ipythonzmq:
+
 Decoupled two-process model
 ==============================