Update and rename README to README.md

Author: pgleeson

README README.mdREADME renamed to README.md

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+## L5 Pyramidal Cell - Mainen et al. 1995
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+[![Continuous build using OMV](https://github.com/OpenSourceBrain/MainenEtAl_PyramidalCell/actions/workflows/omv-ci.yml/badge.svg)](https://github.com/OpenSourceBrain/MainenEtAl_PyramidalCell/actions/workflows/omv-ci.yml)
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 Implementation of the Mainen et al. pyramidal cell model from: Mainen ZF, Joerges J, Huguenard JR, 
 Sejnowski TJ (1995) A model of spike initiation in neocortical pyramidal neurons. Neuron 15:1427-39.