Mention design parameters upfront

Closes #1

README.md

@@ -3,11 +3,11 @@
 ## Overview
 
 Grumpy is a Python to Go source code transcompiler and runtime that is intended
-to be a near drop in replacement for CPython. The key difference is that it
+to be a near drop in replacement for CPython 2.7. The key difference is that it
 compiles Python source code to Go source code which is then compiled to native
 code, rather than to bytecode. This means that Grumpy has no VM. The compiled Go
 source code is a series of calls to the Grumpy runtime, a Go library serving a
-similar purpose to the Python C API.
+similar purpose to the Python C API (although the C API is not directly supported).
 
 ## Running Grumpy Programs