Mention required npx version in readme

Same as rails/jsbundling-rails#9

`set-script` was not part of `npx` until version 7.1.0 (https://github.com/npm/cli/releases/tag/v7.1.0) released December 2020.

Author: t27duck

README.md

@@ -15,7 +15,7 @@ You can configure your bundler options in the `build:css` script in `package.jso
 
 ## Installation
 
-You must already have node and yarn installed on your system. Then:
+You must already have node and yarn installed on your system. You will also need npx version 7.1.0 or later. Then:
 
 1. Add `cssbundling-rails` to your Gemfile with `gem 'cssbundling-rails'`
 2. Run `./bin/bundle install`