added docs/which-hex-file.md to answer the github#1 FAQ

docs/which-hex-file.md

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+# How to figure out which .hex file to use
+
+1. Use the version check function (15 or more clicks from Off) to find out
+   which flavor of the firmware the light has installed.  WRITE THIS DOWN.
+
+2. Look up the model number in the MODELS file, to get the name and MCU type:
+   https://toykeeper.net/torches/fsm/anduril2/MODELS
+
+3. Find the newest firmware for that model.
+   https://toykeeper.net/torches/fsm/anduril2/?C=M;O=D
+
+If the version doesn't include a model number, you may be able to find
+the model in the PRODUCTS file to see which version it probably uses:
+
+  https://bazaar.launchpad.net/~toykeeper/flashlight-firmware/anduril2/view/head:/PRODUCTS
+
+
+## Version Check Formats
+
+The Version Check function should blink out a series of numbers in one of
+several formats:
+
+ - NNNN-YYYY-MM-DD
+   Anduril 2 from 2023-05 or later.
+   It's a model number and build date,
+   with "buzz" flashes between sections.
+   NNNN: model number
+   YYYY: year
+   MM: month
+   DD: day
+
+ - YYYYMMDDNNNN
+   Anduril 2 from 2023-05 or earlier.
+   It's a build date and model number.
+
+ - YYYYMMDD
+   This is an old Anduril 1 version.  It's just a build date.
+   If the model name isn't obvious, try looking it up in the PRODUCTS file.
+