extras/threading_example: add intro

extras/threading_example/threading_example.py

@@ -1,5 +1,24 @@
 """
-This program adds
+This program runs a loop that adds +1 to different cells in an array.
+
+Run it as:
+  python threading_example.py WORKERS ITERATIONS
+where WORKERS is the number of threads,
+and ITERATIONS is how much work each thread does.
+
+In the single threaded case, when N == 1, the 'counters' variable
+below starts out as
+  [0, 0, 0]
+then becomes
+  [1, 0, 0]
+then
+  [1, 1, 0]
+... and a little bit later
+  [2, 1, 1]
+and at the end
+  [N, N, N]
+
+What happens in the multi-threaded case, when N > 1? Try and see!
 """
 
 import sys