Merge pull request #2 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md

README.md

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 ![image](docs/images/GridDyn_FullColor.png "GridDyn")
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 GridDyn is a power system simulator developed at Lawrence Livermore National Laboratory. 
 The name is a concatenation of Grid Dynamics, and as such usually pronounced as "Grid Dine". 
 It was created to meet a research need for exploring coupling between transmission, distribution, and communications system simulations.