KAFKA-104: add documentation on compatibility requirements (#16)

* KAFKA-104: add documentation on compatibility requirements for source and sink connector

Author: Chris Cho

source/index.txt

@@ -39,6 +39,7 @@ This guide is divided into the following topics:
    Source Connector Guide </kafka-source>
    Kafka Docker Example </kafka-docker-example>
    Migrate from Kafka Connect </kafka-connect-migration>
+   Compatibility </kafka-compatibility>
    Changelog <https://github.com/mongodb/mongo-kafka/blob/master/CHANGELOG.md>
 
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source/kafka-compatibility.txt

@@ -0,0 +1,21 @@
+.. _kafka-compatibility:
+
+=============
+Compatibility
+=============
+
+.. default-domain:: mongodb
+
+Kafka Sink Connector
+--------------------
+
+The **Kafka Sink Connector** requires MongoDB 3.6 or later; it is
+unsupported for older versions of MongoDB.
+
+Kafka Source Connector
+----------------------
+
+The **Kafka Source Connector** requires MongoDB 3.6 or later as your data
+source if you are using change streams with a **collection** only. If you 
+need to watch a **database** or **deployment**, you need MongoDB 4.0 or 
+later.