Add a tip to AGENTS.md

Author: fxn

AGENTS.md

@@ -10,6 +10,9 @@ This is the repository for the Ruby gem Zeitwerk, which provides autoloading, ea
 - Study the current implemetation under the `lib` directory.
 - Study the existing test suite under `test`.
 - Execute commands for Ruby tools with `mise exec -- <command>`.
+- It is OK for predicates to return objects different than `true` and `false` if
+  other choices make sense from a pragmatic point of view. Please, check the
+  method `Zeitwerk::Registry.conflicting_root_directory?` for an example.
 
 ## Code style