added readme for toy implementation

toy-impl/README.md

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+# Toy Implementation
+Currently the rough implementation is done, works remain are the code for
+checking a real python script, getting the type variables, some implementation
+details etc.
+
+These features are considered in the proposal, but would not be implemented here
+for simplicity reasons:
+* Referencing Python Variables.
+* range type.
+* Storing large constants as `uint32`, `int64` or `uint64`.
+* AugAssign, `a += b` etc.
+* `with`, `try except`, etc.
+
+These features are currently not implemented, and would be added in due course:
+* Whole script check.
+* Type guards.
+* Primitive type methods and operations.
+* Method override check.
+* Better error messages.
+
+