Update README.md

Author: joecastiglione

README.md

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 ## Welcome
 
-The ActivitySim project emerged from a consortium of Metropolitan Planning Organizations (MPOs) and other transportation planning agencies that wanted to build a shared, open, platform that could be easily adapted to their individual needs, but would share a robust, efficient, and well-maintained common core.
+The mission of the ActivitySim project is to create and maintain advanced, open-source, activity-based travel behavior modeling software based on best software development practices for distribution at no charge to the public.  
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+The ActivitySim project is led by a consortium of Metropolitan Planning Organizations (MPOs) and other transportation planning agencies, which provides technical direction and resources to support project development.  New member agencies are welcome to join the consortium.  All member agencies help make decisions about development priorities and benefit from contributions of other agency partners. 
 
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