Making verbiage consistent w/current _config.yml (mmistakes#3180)

The plugin in question comes by-default listed in _config.yml ; it's better to say that the user must _retain_ it as listed, not that the user needs to add it.

Author: jasonhemann

Diff for: README.md

@@ -15,7 +15,7 @@ Minimal Mistakes is a flexible two-column Jekyll theme, perfect for building per
 [!["Buy Me A Coffee"](https://user-images.githubusercontent.com/1376749/120938564-50c59780-c6e1-11eb-814f-22a0399623c5.png)](https://www.buymeacoffee.com/mmistakes)
  [![Support via PayPal](https://cdn.jsdelivr.net/gh/twolfson/[email protected]/dist/button.svg)](https://www.paypal.me/mmistakes)
 
-**Note:** The theme uses the [jekyll-include-cache](https://github.com/benbalter/jekyll-include-cache) plugin which will need to be installed in your `Gemfile` and added to the `plugins` array of `_config.yml`. Otherwise you'll encounter `Unknown tag 'include_cached'` errors at build.
+**Note:** The theme uses the [jekyll-include-cache](https://github.com/benbalter/jekyll-include-cache) plugin which will need to be installed in your `Gemfile` and must be retained in the `plugins` array of `_config.yml`. Otherwise you'll encounter `Unknown tag 'include_cached'` errors at build.
 
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