readme update

README.md

@@ -16,6 +16,21 @@ the [U2F protocol](https://fidoalliance.org/specifications/overview/).
 Firmware
 ========
 
+### Code overview
+
+The HID layer for U2F is implemented in `u2f_hid.c`.  The USB
+boilerplate is in `callback.c`.
+
+Firmware generally works as follows:
+
+* Main loop adds a request for USB to read a 64 byte packet
+* `USBD_XferCompleteCb` gets called with packet when it comes
+* USB HID layer gets called
+* USB HID layer will call U2F layer when appropriate
+
+[U2F HID layer spec](https://fidoalliance.org/specs/fido-u2f-v1.0-nfc-bt-amendment-20150514/fido-u2f-hid-protocol.html)
+
+[U2F layer spec](https://fidoalliance.org/specs/fido-u2f-v1.0-nfc-bt-amendment-20150514/fido-u2f-raw-message-formats.html)
 
 ### Using Simplicity Studio