Update README.rst - Support for MongoDB 4.0 (mongodb#359)

Author: prayashm

README.rst

@@ -16,7 +16,7 @@ is a `gridfs
 <http://www.mongodb.org/display/DOCS/GridFS+Specification>`_
 implementation on top of ``pymongo``.
 
-PyMongo supports MongoDB 2.6, 3.0, 3.2, 3.4, and 3.6.
+PyMongo supports MongoDB 2.6, 3.0, 3.2, 3.4, 3.6 and 4.0.
 
 Support / Feedback
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