Add plugins constant

There was some confusion with the plugins directory in Chassis for @highergroundstudio. See https://github.com/Chassis/Chassis/issues/98

I've updated the README to include the creation of `plugins` and `themes` directories and I've defined both `WP_PLUGIN_DIR` and `WP_PLUGIN_URL`. Can you see any issues with this going forward @rmccue? #ReviewMerge