add gmail_api example to README.md (#518)

docs/source/example.ini

@@ -21,4 +21,11 @@ index = email
 
 [s3]
 bucket = my-bucket
-path = parsedmarc
+path = parsedmarc
+
+[gmail_api]
+credentials_file = /etc/example/credentials.json
+token_file = /etc/example/token.json
+include_spam_trash = True
+paginate_messages = True
+scopes = https://www.googleapis.com/auth/gmail.modify

docs/source/usage.md

@@ -317,6 +317,10 @@ The full set of configuration options are:
       credentials, None to disable (Default: `None`)
   - `token_file` - str: Path to save the token file
       (Default: `.token`)
+      
+    :::{note}
+    credentials_file and token_file can be got with [quickstart](https://developers.google.com/gmail/api/quickstart/python).Please change the scope to `https://www.googleapis.com/auth/gmail.modify`.
+    :::
   - `include_spam_trash` - bool: Include messages in Spam and
       Trash when searching reports (Default: `False`)
   - `scopes` - str: Comma separated list of scopes to use when