added notes about numeric types

README.md

@@ -58,13 +58,14 @@ class Foo(EnvExperiment):
 ## Built-in Types
 * Primitive types include:
     * `bool`
+    * `byte`
     * `int32`
     * `int64`
     * `uint32`
     * `uint64`
     * `float`
     * `str` (note: fixed length, provide builtin methods?)
-    * `bytes`
+    * `bytes` (a list of `byte`, but with more convenient syntax)
 * Collections include:
     * `list`: homogeneous (elements must be of the same type) fixed-size (no
       append) list.
@@ -83,6 +84,16 @@ class Foo(EnvExperiment):
       ```
     * `range` (over numerical types)
 
+### Numerical Types
+* All binary operations expect the values to have the same type, no implicit
+  coercion would be performed, explicit casting is required.
+* Casting can be done by `T(v)` where `T` is the target type, and `v` is the
+  original value. Examples: `int64(123)`
+* Constant integers are treated as `int32` by default. If the value cannot
+  be stored in `int32`, `uint64` would be used if the integer is non-negative,
+  and `int64` would be used it the integer is negative.
+* Only `uint32`, `int32` (and range of them) can be used as index.
+
 ## Generics
 We use [type variable](https://docs.python.org/3/library/typing.html#typing.TypeVar) for denoting generics.