Add a sentence about GitHub Copilot reviews

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: kou

docs/source/developers/overview.rst

@@ -104,6 +104,10 @@ When contributing a patch, use this list as a checklist of Apache Arrow workflow
   merged, this will be retained in the extended commit message.
 * Make sure that your code **passes the unit tests**. You can find instructions how
   to run the unit tests for each Arrow component in its respective README file.
+* In order to improve our code and your review experience, we have
+  enabled GitHub Copilot reviews to automate the first pass. Please
+  address all GitHub Copilot comments by either implementing them, or
+  explicitly responding and explaining why you chose not to.
 
 Core developers and others with a stake in the part of the project your change
 affects will review, request changes, and hopefully indicate their approval