Document --dump-default-config in README.md

Fixes #317

Author: ssilva

README.md

@@ -209,6 +209,8 @@ options covering different styles. File an issue, or even better, submit a PR.
 * When you run rustfmt, place a file named `rustfmt.toml` or `.rustfmt.toml` in
   target file directory or its parents to override the default settings of
   rustfmt.
+* You can dump the default configuration with
+  `rustfmt --dump-default-config rustfmt.toml` and customize it as needed.
 * After successful compilation, a `rustfmt` executable can be found in the
   target directory.
 * If you're having issues compiling Rustfmt (or compile errors when trying to