Clarified spi_callback functionality. Part of #5

datagram.md

@@ -4,6 +4,7 @@ Unlike the remote api with its many functions, this has only two, send and recei
 You must write a custom program running on the AMMDK k66f to make any practical use of this functionality.
 
 `test/blip-test.cpp` is an example of a program that uses this API.
+It requires `heartrateLED.elf` from the `amm-tiny` repo to be on the k66f.
 
 ## Functions
 
@@ -13,6 +14,7 @@ You must write a custom program running on the AMMDK k66f to make any practical
 * -1 if `len` > `SPI_MSG_PAYLOAD_LEN`
 * -2 if no space available to send message
 
-`TODO` is a callback for received messages.
+The `spi_callback` variable is a callback for received messages.
+If you do not need to receive messages, it can be `NULL` but it must be present.
 
 `void datagram_task(void)` should be called as a thread, like `std::thread thread(datagram_task);` and must be running before `send_message` is called.