docs: add description to extendable eslint configs [ci skip] (#1862)

Author: st-sloth

README.md

@@ -1,3 +1,11 @@
 # @vue/eslint-config-standard
 
 > eslint-config-standard for vue-cli
+
+This config is specifically designed to be used by `vue-cli` setups
+and is not meant for outside use (it can be used but some adaptations
+on the user side might be needed - for details see the config file).
+
+A part of its design is that this config may implicitly depend on
+other parts of `vue-cli` setups, such as `eslint-plugin-vue` being
+extended in the same resulting config.