Update README with the 'custom-' prefix requirement

README.md

@@ -357,13 +357,14 @@ local builds of the Rust toolchain. To teach `rustup` about your build
 just run:
 
 ```
-$ rustup toolchain link my-toolchain path/to/my/toolchain/sysroot
+$ rustup toolchain link custom-my-toolchain path/to/my/toolchain/sysroot
 ```
 
-Now you can name `my-toolchain` as any other `rustup`
+(The name for a custom toolchain _must_ have the `custom-` prefix.)
+Now you can name `custom-my-toolchain` as any other `rustup`
 toolchain. Create a `rustup` toolchain for each of your
 `rust-lang/rust` workspaces and test them easily with `rustup run
-my-toolchain rustc`.
+custom-my-toolchain rustc`.
 
 Because the `rust-lang/rust` tree does not include Cargo, *when `cargo`
 is invoked for a custom toolchain and it is not available, `rustup`