notes about optional parameter value

README.md

@@ -51,6 +51,17 @@ class Foo(EnvExperiment):
 * No implicit coercion, require implicit cast. Integers are int32 by default,
   floating point numbers are double by default.
 * RPCs: optional parameter type signature, require return type signature.
+* Value of the optional parameter would be created by the caller, and
+  independent between different invokations. The following code would always
+  output 2 `1`, instead of `1` and `2` for actual Python:
+  ```py
+  def test(a = [1]):
+      print(a)
+      a[0] += 1
+
+  test()
+  test()
+  ```
 
 ## Generics
 We use [type variable](https://docs.python.org/3/library/typing.html#typing.TypeVar) for denoting generics.