Update README.md

document the serial commands that the DTMF Shield implements

README.md

@@ -42,3 +42,20 @@ the app on windows, and run it under Mono.  However, the Mac and Linux seem to u
 works that way.  It looks like the way forward is to convert to a Xamarin app.  But I could be very wrong, and chasing alleged unicorns.
 
 I have and use MacOS and Linux myself, but help is appreciated!
+
+
+What the DTMF_Shield sketch does:
+1) it processes a set of serial command strings.  Some are single character, some are multiple character.  These commands/actions are described below.
+2) to operate the shield itself, either a command setting a value is sent, a button press is emulated, or a 5 bit command is sent to the DTMF generator chip (HT9200A)
+  to generate a tone for 0-9 or A-D.	The chip is enabled, the 5 bits are banged out, and the chip is disabled.  There are appropriate pauses.
+	 a) setting values: accepts "SET_TONE_TIME:" followed by an integer value, or "SET_BUTTON_TIME:" followed by an integer value, "PAUSE: ", which causes a pause for
+     TONE_DELAY time (SET_TONE_TIME value)
+	 b) receiving the string "0" thru "9" and "A" thru "D" cause that DTMF tone to be generated by the HT9200A chip, for the current TONE_DELAY time.
+	 c) receiving the string "UP" causes the UP button press line to be set for BUTTON_DELAY time (SET_BUTTON_TIME value), then cleared.
+    Similar for "DOWN", "MR", "VFO", and "CALL"
+
+	Long and short presses are performed by having the TM742 programmer application change the TONE_TIME and BUTTON_TIME appropriately,
+ then returning to the normal value.
+	A possible improvement could to build in things like 'short MR' and 'long MR' by having more time values that can be set.  But those actions can already be
+ performed with	the commands that are now present.
+