Readme: make free account type explicit for 500k free characters

Author: daniel-jones-dev

README.md

@@ -18,8 +18,8 @@ after they’re added to the API.
 ## Getting an authentication key
 
 To use the DeepL Python Library, you'll need an API authentication key. To get a
-key, [please create an account here][create-account]. You can translate up to
-500,000 characters/month for free.
+key, [please create an account here][create-account]. With a DeepL API Free
+account you can translate up to 500,000 characters/month for free.
 
 ## Installation