doc: update readme with upgrading instructions, #205

README.md

@@ -147,6 +147,15 @@ end
 
 This gem is tested against currently supported Ruby and Rails versions. For an up to date list, check the build matrix in the [workflow](.github/workflows/ci.yaml).
 
+## Upgrading to v5.3.0
+
+In version v5.3.0 the config directory files were renamed as follows:
+
+`config/blacklisted_email_domains.yml` -> `config/deny_listed_email_domains.yml`
+`config/whitelisted_email_domains.yml` -> `config/allow_listed_email_domains.yml`
+
+You won't need to make any changes yourself if you're installing this version for the first time. For individuals updating from earlier versions, make sure to update the file namings as per the above. In future versions this will be a breaking change.
+
 ## Upgrading to v3.0.0
 
 In version v3.0.0 I decided to move __and__ rename the config files from the