Add more info on useful workflows

[mark-i-m]

• x.py with ra: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/IDE/near/197315300
• git worktree: https://rust-lang.zulipchat.com/#narrow/stream/196385-t-compiler.2Fwg-rustc-dev-guide/topic/traits.2Fcanonicalization/near/196914317