update readme

Readme states, that current version (unspecified) is compatible with bevy 0.14. However newest version of bevy is 0.15. When checking `Cargo.toml` I found out that current version (0.22) has specified dependency of Bevy 0.15, thus we should be compatible with bevy 0.15. I've updated README.md to state `big_brain`'s version 0.22 is compatible with bevy 0.15