Add snippet illustrating how to use time left

docs/snippet_timeleft.cpp

@@ -0,0 +1,36 @@
+/*
+
+This is a code snippet for how to use the time remaining on the timer in your puzzle module.
+
+To avoid the time left getting stale and inaccurate, we instead calculate when the timer will run
+out and store this deadline. When we need to get the time left, we can then calculate it.
+
+This does not handle some edgecases:
+- times and timedeltas are stored in a 32-bit unsigned int, this can overflow after ~50 days, causing all sorts of weirdness
+ => avoid having modules powered on for a long time
+- the get_millis_remaining() getting called before receiving the first state update
+ => try to only call this function after human input
+ => alternatively, keep track of whether you can call the `get_millis_remaining` function already in a boolean
+*/
+
+uint32_t deadline;
+
+
+void callback_state(uint32_t time_left, uint8_t strikes, uint8_t max_strikes, uint8_t puzzle_modules_left) {
+	// Mark arguments as not used
+	(void)strikes;
+	(void)max_strikes;
+	(void)puzzle_modules_left;
+
+	deadline = millis() + time_left;
+}
+
+uint32_t get_millis_remaining() {
+	return deadline - millis();
+}
+
+void loop() {
+	/* ... */
+	bool is_message_valid = obus_module::loopPuzzle(&message, callback_game_start, callback_game_stop, empty_callback_info, callback_state);
+	/* ... */
+}