Add note to README.md about flatc

TensorFlow v2.18.1 requires flatc host tool for cross compilation. Add a
note stating which version has been tested, to simplify user setup.

Change-Id: I39bdd318383e01c671f3238a52607d0d4ad91c21
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>