Explain that checked can be a footgun in React

I ended up learning this on my own during the exercise, but now that
I've arrived at the "Default Value" lesson I'm seeing this is already
included in the course material.

Adding a call-out for it is kind of nice, especially since the
instructions start to talk about `checked` but don't explain the
read-only part.

Author: ianjmacintosh

exercises/08.inputs/03.problem.radio/README.mdx

@@ -13,8 +13,12 @@ multiple inputs that share the same name. Only one of them can be selected at
 a time. Each radio input needs its own `label`. To label the entire group, you
 can use a `<fieldset>` and a `<legend>`.
 
-Similar to checkboxes, to set the default value, you need to add the `checked`
-attribute to the input you want to be selected by default.
+<callout-info>
+	Note: Similar to checkboxes, to set the default value, you can set the
+	`checked` attribute to the input you want to be selected by default... but
+	in React, that will make your input read-only! We'll learn more about how
+	to set default values in React in a future lesson.
+</callout-info>
 
 Similar to a select `option`, you need to set the `value` on the input to
 determine what value will be sent to the server when the form is submitted.