Update README.md

quad/src/quad_app/README.md

@@ -20,6 +20,23 @@ One potential confusing point to take note of is the difference between "(X/Y) V
 
 * Note our use of derivative on value, not error
 
+## What about the Brauns?
+We said that the application is the brains, and it is just that. It computes
+outputs from inputs, but it has no idea where those inputs come from, or where
+its outputs should go.
+
+Specifically, the application has 12 unimplemented interfaces, defined in
+[`hw_iface.h`](hw_iface.h). This design allows us to run the quad application
+on whatever hardware we want, we just have to implement the interfaces.
+
+![interface diagram](../../doc/images/application_platform_interface.png)
+
+We have 2 primary platforms on which we run the quad application:
+1. [Zybo board (Real Quad)](../../xsdk_workspace/real_quad/README.md)
+  - Used to actually fly the quad
+2. [Unix-like computer (Virtual Quad)](../../src/virt_quad/README.md)
+  - Used in automated testing and to ease ground station development
+
 ## Optical Flow
 The current (end of may17-16) state of optical flow is that it can be used autonomously be uncommenting all four defines at the top of `control_algorithm.c`. It is relatively stable, but drifts over time. Setpoints do work, but yaw needs fixing (See issue #23).