tutorial: clarify that `rustup install nightly` doesn't change the default
@@ -78,6 +78,8 @@ First install Jujutsu:
```shell script
$ git clone https://github.com/martinvonz/jj.git jj-git
$ cd jj-git
+# We need the "nightly" Rust toolchain. This command installs that without
+# changing your default.
$ rustup install nightly
$ cargo +nightly install --path .
$ cd ..
