rust: MAINTAINERS: Add the zulip link

Zulip organization "rust-for-linux" was created 2 years ago[1] and has
proven to be a great place for Rust related discussion, therefore
add the information in MAINTAINERS file so that newcomers have more
options to find guide and help.

Link: https://lore.kernel.org/rust-for-linux/CANiq72=xVaMQkgCA9rspjV8bhWDGqAn4x78B0_4U1WBJYj1PiA@mail.gmail.com/

 [1]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Alice Ferrazzi <alice.ferrazzi@miraclelinux.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>