Update README.

Author: m-ou-se

tools/unstable-api/README.md

@@ -7,7 +7,7 @@ This tool will dump the public API for an unstable feature.
 From this directory, run something like:
 
 ```shell
-cargo run --release -- --feature $feature --repo-root $path_to_rust | rustfmt
+cargo run --release -- --feature $feature --repo-root $path_to_rust
 ```
 
 where `$feature` is the name of the unstable feature and `$path_to_rust` is the path to a local clone of `rust-lang/rust`. You'll probably need to run `x.py` first to make sure submodules are cloned.
@@ -16,9 +16,16 @@ You can also install it as a Cargo tool and use it that way:
 
 ```shell
 cargo install --path .
-cargo unstable-api --feature $feature --repo-root $path_to_rust | rustfmt
+cd $path_to_rust
+cargo unstable-api --feature $feature
 ```
 
+You can leave out the `--repo-root` option when running it inside the `rust-lang/rust` repository.
+
+## Output formatting
+
+On Unix, the output is automatically formatted with `rustfmt` and highlighted with `bat`, if you have those tools installed.
+
 ## Limitations
 
 APIs that are generated by macros won't be picked up. This tool just parses the source code so it can be run without needing to build the standard library itself.