Add compatibility guide (#316)

docs/source/compatibility.md

@@ -0,0 +1,40 @@
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+  Licensed to the Apache Software Foundation (ASF) under one
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+  "License"); you may not use this file except in compliance
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+    http://www.apache.org/licenses/LICENSE-2.0
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+-->
+
+# Compatibility Guide
+
+Comet aims to provide consistent results with the version of Apache Spark that is being used.
+
+This guide offers information about areas of functionality where there are known differences.
+
+## ANSI mode
+
+Comet currently ignores ANSI mode in most cases, and therefore can produce different results than Spark. By default,
+Comet will fall back to Spark if ANSI mode is enabled. To enable Comet to accelerate queries when ANSI mode is enabled,
+specify `spark.comet.ansi.enabled=true` in the Spark configuration. Comet's ANSI support is experimental and should not
+be used in production.
+
+There is an [epic](https://github.com/apache/datafusion-comet/issues/313) where we are tracking the work to fully implement ANSI support.
+
+## Cast
+
+Comet currently delegates to Apache DataFusion for most cast operations, and this means that the behavior is not 
+guaranteed to be consistent with Spark.
+
+There is an [epic](https://github.com/apache/datafusion-comet/issues/286) where we are tracking the work to implement Spark-compatible cast expressions.

docs/source/index.rst

@@ -43,6 +43,7 @@ GitHub repository at https://github.com/apache/datafusion-comet.
    :maxdepth: 1
    :caption: Project Links
 
+   compatibility
    Github and Issue Tracker <https://github.com/apache/datafusion-comet>
 
 .. _toc.asf-links: