Include testlib sync policy in the README.

I wrote a brief note about keeping testlib.sml in sync for all
exercises in the track, and I mentioned 'make redeploy-testlib' as the
provided tool for handling this requirement.

I did not elaborate on a policy, because we still don't really have
one.  Further work related to exercism#110 should result in a clear policy,
and at that point the README.md might need some more updating.

README.md

@@ -30,7 +30,11 @@ Every exercise must have at least these files:
 
 ### `testlib.sml`
 
-This helper has this structures:
+The copy of `testlib.sml` for each exercise should be in sync with `lib/testlib.sml`.
+`make redeploy-testlib` is provided for syncing all exercises with `lib/testlib.sml`
+when it is updated.
+
+This helper has these structures:
 
 ```sml
 structure Expect: