README does not mention Values

The [readme](https://github.com/css-modules/css-modules-loader-core/tree/b99eaee56f44ad51e38da4b9e8ffd1dba2ec3e82#new-coreplugins) seems to have not been updated to mention that [postcss-modules-values](https://github.com/css-modules/postcss-modules-values) is now included in the default set of core plugins.  This caused me some troubles when I tried to manually run it after the core plugins.
