Adding a complete description of the IPython security model.

docs/source/parallel/index.txt

@@ -11,4 +11,5 b' Using IPython for parallel computing'
    parallel_multiengine.txt
    parallel_task.txt
    parallel_mpi.txt
+   parallel_security.txt
 

docs/source/parallel/parallel_intro.txt

@@ -148,6 +148,9 b' ipcontroller-mec.furl'
 
 More details of how these ``.furl`` files are used are given below.
 
+A detailed description of the security model and its implementation in IPython
+can be found :ref:`here <parallelsecurity>`.
+
 Getting Started
 ===============