added builtin types section

README.md

@@ -55,6 +55,32 @@ class Foo(EnvExperiment):
   would not be kept across kernel calls, and that is a potential source of
   confusion.
 
+## Built-in Types
+* Primitive types include:
+    * `bool`
+    * `int32`
+    * `int64`
+    * `float`
+    * `str` (note: fixed length, provide builtin methods?)
+    * `bytes`
+* Collections include:
+    * `list`: homogeneous (elements must be of the same type) fixed-size (no
+      append) list.
+    * `tuple`: inhomogeneous fixed-size list, only pattern
+      matching (e.g. `a, b, c = (1, True, 1.2)`) and constant indexing
+      is supported:
+      ```
+      t = (1, True)
+      # OK
+      a, b = t
+      # OK
+      a = t[0]
+      # Not OK
+      i = 0
+      a = t[i]
+      ```
+    * `range` (over `int32`, `int64`, `float`)
+
 ## Generics
 We use [type variable](https://docs.python.org/3/library/typing.html#typing.TypeVar) for denoting generics.