message: Add note about .cargo/config support.
diff: @@ -30,6 +30,11 @@ together. Numbers, strings, and booleans will use the value in the deeper
  config directory taking precedence over ancestor directories, where the
  home directory is the lowest priority. Arrays will be joined together.
  +> **Note:** Cargo also reads config files without the `.toml` extension, such as
  +> `.cargo/config`. Support for the `.toml` extension was added in version 1.39
  +> and is the preferred form. If both files exist, Cargo will use the file
  +> without the extension.
  +
  ### Configuration format
  Configuration files are written in the [TOML format][toml] (like the
  