Update README, differentiate better between flight core and middleware in docs.

README.md

-## PX4 Flight Control Stack and Middleware ##
+## PX4 Flight Core and PX4 Middleware ##
 
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+This repository contains the PX4 Flight Core, with the main applications located in the src/modules directory. It also contains the PX4 Drone Platform, which contains drivers and middleware to run drones. 
+
 *   Official Website: http://px4.io
 *   License: BSD 3-clause (see LICENSE.md)
 *   Supported airframes (more experimental are supported):