add note on ivars

README.md

@@ -60,6 +60,14 @@ it 'should transition nowhere' do
 end
 ```
 
+## Note on instance variables
+
+Currently, storing state in an instance variable is not supported. This due to
+the fact that instance variables cannot be observed for changes like an
+accessor can. For more information, see [the tests](https://github.com/keygen-sh/transition_through/blob/d2c6b685e4959d08e70bb5012af98fa01fcdebef/spec/transition_through_spec.rb#L8-L21).
+
+If you know of a way to support ivars, please open an issue or PR.
+
 ## Supported Rubies
 
 **`transition_through` supports Ruby 3.1 and above.** We encourage you to upgrade