Update README.md

Author: pgleeson

README.md

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-Hodgkin Huxley Tutorial
-======================
+### Hodgkin Huxley Tutorial
 
 This tutorial contains a side-by-side comparison of Python code that runs the [Hodgkin-Huxley equations](https://en.wikipedia.org/wiki/Hodgkin%E2%80%93Huxley_model) and generates plots with a [NeuroML2 implementation](http://journal.frontiersin.org/Journal/10.3389/fninf.2014.00079/abstract) of the same equations.