Explain how to install rustfmt

Author: da-x

README.md

@@ -73,7 +73,8 @@ configure Tagbar to some degree.
 ### Formatting with [rustfmt][rfmt]
 
 The `:RustFmt` command will format your code with
-[rustfmt][rfmt] if installed.
+[rustfmt][rfmt] if installed. `rustfmt` can be installed
+via `rustup component add rustfmt`.
 
 Placing `let g:rustfmt_autosave = 1` in your `~/.vimrc` will
 enable automatic running of `:RustFmt` when you save a buffer.