Add R9 - Plan 9 kernel reimplementation in Rust

Kernel: https://github.com/r9os/r9
Potential file system: https://github.com/google/rust-p9

Architecture: aarch64, riscv64, x86_64
Pure Rust: no
Active: yes
Kernel Architecture: p9
Target: ?
User Space: ?
Optional GUI: no
Contributors: 10
File System: n/a - seems like just kernel, can likely use an implementation of plan nine fs like rust-p9
License: MIT
File System License: BSD 3-Clause