docs: Update README with a bit more details about available flags

Fixes #1285.

@cushon

Fixes #1288

FUTURE_COPYBARA_INTEGRATE_REVIEW=#1288 from vorburger:patch-8 6573012
PiperOrigin-RevId: 811850820

Author: vorburger

README.md

@@ -26,6 +26,15 @@ The formatter can act on whole files, on limited lines (`--lines`), on specific
 offsets (`--offset`), passing through to standard-out (default) or altered
 in-place (`--replace`).
 
+Option `--help` will print full usage details; including built-in documentation
+about other flags, such as `--aosp`, `--fix-imports-only`,
+`--skip-sorting-imports`, `--skip-removing-unused-import`,
+`--skip-reflowing-long-strings`, `--skip-javadoc-formatting`, or the `--dry-run`
+and `--set-exit-if-changed`.
+
+Using `@<filename>` reads options and filenames from a file, instead of
+arguments.
+
 To reformat changed lines in a specific patch, use
 [`google-java-format-diff.py`](https://github.com/google/google-java-format/blob/master/scripts/google-java-format-diff.py).