Add README.

README.md

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+# Typed Interface for the Generic Graph Library
+
+This module provides a typed interface to the (generic graph
+library)[https://docs.racket-lang.org/graph/index.html].
+
+This module is currently **work in progress** and multiple planned
+features may still be missing.
+
+For more details, refer to the Scribble documentation of the package.