Added readme, too

README.md

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+# `activitystreams-aeson`
+
+A Haskell interface to [http://activitystrea.ms/](Activity Streams); in
+particular, the [http://activitystrea.ms/specs/json/1.0/](Activity
+Streams 1.0 RFC) and the [https://github.com/activitystreams/activity-schema/blob/master/activity-schema.md](proposed base schema).