Add a success indication

Also document the command.

README.md

@@ -72,9 +72,10 @@ Without it, the expiry action will be taken.
 Command is one of:
 
 | Command    | Description                                                                                                | 
-|---------|------------------------------------------------------------------------------------------------------------|
+|------------|------------------------------------------------------------------------------------------------------------|
 | check      | Print a list of revisions and notes that would be expired, based on the given parameters for `-n` and `-r` |
 | cron       | Run `expire` at 2 am local time each day. Will run until killed.                                           |
+| emailcheck | Send a test email from the configured sender to themselves.                                                |
 | expire     | Expire old revisions and notes.                                                                            |
 
 ### Environment variables

hedgedoc-expire.py

@@ -69,6 +69,7 @@ class EmailSender:
         except Exception as e:
             print(f'Unable to send mail through {self}: {e}')
             raise e
+        print(f'Test email to {self.mail_from} sent successfully.')
 
     def __str__(self):
         return f'EmailSender<{self.hostname},{self.port},{self.username},{self.mail_from}>'
@@ -291,6 +292,7 @@ def main():
         command is one of:
          - check: print a list of revisions and notes to be expired
          - cron: run expire every 24 hours
+         - emailcheck: send a test email from the configured sender to themselves
          - expire: expire old revisions and untouched notes
         
         See https://git.hamburg.ccc.de/CCCHH/hedgedoc-expire