Add a TODO

I know the current value is wrong...

Change-Id: I574f4c0d3098549f91e536e5ae9a983906684d4a
diff --git a/y2017/constants.h b/y2017/constants.h
index 3ca12c9..4d31b3c 100644
--- a/y2017/constants.h
+++ b/y2017/constants.h
@@ -20,6 +20,7 @@
   static const int kZeroingSampleSize = 200;
 
   // The ratio from the encoder shaft to the drivetrain wheels.
+  // TODO(constants): Fix this.
   static constexpr double kDrivetrainEncoderRatio =
       (18.0 / 36.0) /*output reduction*/ * (44.0 / 30.0) /*encoder gears*/;