make a warning to use PyMC3 instead.

readme.rst

@@ -2,6 +2,8 @@
  Monte-Carlo Experiment
 ***********************
 
+NOTE: This an experimental package is only of historical interest. It formed the basis of what is now [PyMC3](https://github.com/pymc-devs/pymc3). Use that instead.  
+
 MCEx is an experimental package designed to be allow experimentation with MCMC package design. 
 It's goal is to be simple to use, understand, extend and improve, while still being fast. 
 The hope is that some of the lessons learned in this experimental package lead to improvements
@@ -50,4 +52,4 @@ Possible Changes
 
  * Make HMC and related automatically choose a variance/covariance
  * Eliminate `model` arguments to AddVar, AddData, step methods etc by making it hidden and default
- * Eliminate test = argument to AddVar by allowing the user to specify a 'test point'. 
+ * Eliminate test = argument to AddVar by allowing the user to specify a 'test point'.