Hint "rust-analyzer.rustcSource": "discover" next to rustc_private = true. (#825)

Author: eddyb

crates/rustc_codegen_spirv/Cargo.toml

@@ -51,6 +51,8 @@ pipe = "0.4"
 pretty_assertions = "1.0"
 tempfile = "3.2"
 
+# Note that in order to use RA and have access to `rustc_*` crates, you also
+# need to set `"rust-analyzer.rustcSource": "discover"` in e.g. VSCode.
 [package.metadata.rust-analyzer]
 # This crate uses #[feature(rustc_private)]
 rustc_private = true