quad: Add docstring for hw_iface

quad/src/quad_app/hw_iface.h

 #ifndef HW_IFACE_H
 #define HW_IFACE_H
 
+/**
+ * Hardware Interfaces
+ *
+ * These interfaces are used to accomplish separation between the application
+ * layer and the hardware layer in the program that runs on the quadcopter.
+ *
+ * NOTE:
+ * If you wound up here after following some IDE function declaration trail,
+ * you've hit the end of the application layer. Go to the location of the
+ * hardware layer appropriate for your circumstance:p
+ * ../../xsdk_worksapce/modular_quad_pid -> running quad_app on the Zybo
+ * ../virt_quad -> running quad_app in a Unix environment
+ */
+
 struct I2CDriver {
   void *state;
   int (*reset)(struct I2CDriver *self);