message: Add documentation about target.$triple.runner
diff: @@ -66,6 +66,11 @@ vcs = "none"
  linker = ".."
  # Same but for the library archiver which is passed to rustc via `-C ar=`.
  ar = ".."
  +# If a runner is provided, compiled targets for the `$triple` will be executed
  +# by invoking the specified runner executable with actual target as first argument.
  +# This applies to `cargo run`, `cargo test` and `cargo bench` commands.
  +# By default compiled targets are executed directly.
  +runner = ".."
  # custom flags to pass to all compiler invocations that target $triple
  # this value overrides build.rustflags when both are present
  rustflags = ["..", ".."]
  