Add a roadmap skeleton

ROADMAP.md

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+# smithy-python interfaces
+* Data Class Generation
+    * Scalar Values
+    * Enums
+    * Structures
+    * List
+    * Set
+    * Union
+    * Documents
+    * Exceptions
+* HTTP Client Interfaces
+    * Request / Response data classes
+    * Mypy Protocol Definitions for HTTP client/session
+* Middleware Stack
+* Service Client Generation
+* Operation Interface Generation
+* Paginators
+* Waiters
+* Event Streams
+
+# Standalone
+* CRT based HTTP client that implements Smithy HTTP Client Interface
+* AWS signing package
+    * SigV4
+    * SigV4a
+* AWS credential resolution package
+
+# aws-python-sdk
+* Endpoint Resolution
+* Marshalling Generation
+    * Serialization (Query, JSON, RestJSON, RestXML)
+    * Deserialization (Query, JSON, RestJSON, RestXML)
+* Credential Resolution
+* Client Configuration