Add state diagram

Author: ErikSchierboom

exercises/split-second-stopwatch/instructions.md

@@ -9,3 +9,25 @@ The stopwatch has the following functionality:
 - Previous laps: retrieve the previously recorded lap times
 - Reset: stop time tracking, reset the current lap, and clear all previously recorded laps
 - Lap: add the current lap time to the previous laps, then reset the current lap time and continue tracking time
+
+You can think of a stopwatch as being in one of three states:
+
+1. Ready
+2. Running (tracking time)
+3. Stopped (not tracking time)
+
+This is a visualization of the states (between square brackets) and the commands (between angular brackets):
+
+```text
+                     <lap>
+                    +-----+
+                    |     |
+         <start>    v     |   <stop>
+[Ready] ---------> [Running] --------> [Stopped]
+   ^            ^                        |   |
+   |            |        <start>         |   |
+   |            +------------------------+   |
+   |                                         |
+   |                <reset>                  |
+   +-----------------------------------------+
+```