message: document new feature
diff: @@ -308,3 +308,15 @@ $ cargo +nightly build -Z compile-progress
  Compiling utf8-ranges v1.0.0
  Building [=======> ] 2/14: libc, regex, uc...
  ```
  +
  +### default-run
  +* Original issue: [#2200](https://github.com/rust-lang/cargo/issues/2200)
  +
  +The `default-run` option in the `[project]` section of the manifest can be used
  +to specify a default binary picked by `cargo run`. For example, when there is
  +both `src/bin/a.rs` and `src/bin/b.rs`:
  +
  +```toml
  +[project]
  +default-run = "a"
  +```
  