add Binder button

Author: labarba

README.md

@@ -19,6 +19,9 @@ In all cases, learners should follow along the worked examples in each lesson by
 
 Lessons
 -------
+> Launch an interactive session with this module using the Binder service:
+[![Binder](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/barbagroup/CFDPython/master)
+
 Steps 1–4 are in one spatial dimension. Steps 5–10 are in two dimensions (2D). Steps 11–12 solve the Navier-Stokes equation in 2D. Three "bonus" notebooks cover the CFL condition for numerical stability, array operations with NumPy, and defining functions in Python.
 
 * [Quick Python Intro](http://nbviewer.jupyter.org/github/barbagroup/CFDPython/blob/master/lessons/00_Quick_Python_Intro.ipynb)