add information about flow typed files

[buschco]

Checklist:

• [x] All tests pass in CI.
• [x] There are sufficient tests for the new fix/feature.
• [x] Grammar rules have not been renamed unless absolutely necessary.
• [x] The conflicts section hasn't grown too much.
• [x] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

Just a small readme update.

[maxbrunsfeld]

Thanks!