additional warning

Co-authored-by: Mason Protter <[email protected]>

Author: nsajko

README.md

@@ -35,6 +35,7 @@ Note regarding the return type of `@spawn`:
     * the configuration of type inference in the Julia compiler
     * the return type of the input function
         * in particular, `typeof(StableTasks.@spawn 3) != typeof(StableTasks.@spawn 3.0)`, because `typeof(3) !== typeof(3.0)`
+* You should **not** write code that relies upon the exact return type of `StableTasks.@spawn` being concrete, or stable across versions. The return type in its parameter is available **only** as an optimization.
 
 ## `StableTasks.@spawnat`