Added note about experimental Evaluator commands

See #502

docs/pages/7 - Extending Mill.md

@@ -158,7 +158,9 @@ builds, simply publish your code as a library to maven central.
 For more information, see Ammonite's
 [Ivy Dependencies documentation](http://ammonite.io/#import$ivy).
 
-## Evaluator Commands
+## Evaluator Commands (experimental)
+
+_Evaluator Command are experimental and suspected to change. See [issue #502](https://github.com/lihaoyi/mill/issues/502) for details._
 
 You can define a command that takes in the current `Evaluator` as an argument,
 which you can use to inspect the entire build, or run arbitrary tasks. For
@@ -179,3 +181,4 @@ Many built-in tools are implemented as custom evaluator commands:
 [all](http://www.lihaoyi.com/mill/#all), [inspect](http://www.lihaoyi.com/mill/#inspect),
 [resolve](http://www.lihaoyi.com/mill/#resolve), [show](http://www.lihaoyi.com/mill/#show).
 If you want a way to run Mill commands and programmatically manipulate the tasks and outputs, you do so with your own evaluator command.
+