Add README file

COUCHDB-2539
diff --git a/README.rst b/README.rst
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+==============
+couchdb-b64url
+==============
+
+This is a pretty simple NIF that is just responsible for encoding and decoding
+Base46 URL values::
+
+    1> Thing = b64url:encode("Hello, CouchDB!").
+    <<"SGVsbG8sIENvdWNoREIh">>
+    2> b64url:decode(Thing).
+    <<"Hello, CouchDB!">>
+
+License
+=======
+
+Apache 2.0