Explain the auto-formatter

Author: pavpanchekha

AGENTS.md

@@ -6,5 +6,7 @@
   `ops/`; the real evaluator is in `eval/`; the REPL is in `repl.rkt`.
   The tests are in `test.rkt`; the nightly is `time.rkt`. The nightly
   also has various helper functions in `infra/`
+- Mandatory auto-format. Always run `make fmt` before finishing your
+  task to format your code correctly.
 - When editing an interval arithmetic function, always run the tests.
   Run tests with `racket -y test.rkt add` (for the right function).