Update AGENTS.md to include test method documentation requirement

Author: erict875

AGENTS.md

@@ -18,6 +18,7 @@ A simple Java client for ACINQ's [phoenixd REST API](https://phoenix.acinq.co/se
 - Add integration tests for new features to verify end-to-end functionality.
 - Ensure new dependencies or configurations do not introduce security vulnerabilities.
 - Maintain the versions in the configuration section of the pom.xml files.
+- Add a comment on top of every test method to describe the test in plain English.
 
 ## Pull Requests