Add reference to `rusttest-to-dg` tool

[MahadMuhammad]

          Later we'll need to mention where could people find this tool.

Originally posted by @P-E-P in https://github.com/Rust-GCC/gccrs/pull/3080#discussion_r1691088628

Ref: https://github.com/Rust-GCC/rusttest-to-dg