Update README

* Add link to new blog post
Fix mail service bug

* Recipients for emails are filtered now: only persons that have an
  email address may be recipients. This avoids that exceptions are
  thrown during mime message preparation.

* refs #18195
Avoid setting variable in custom number tag

* Setting a variable avoids that the formatted number is shown when
  using the custom number tag.

* refs #18151
Rename naming for overtime vacation type

* refs #18152
Update role descriptions in person form

* Some clean up in `messages.properties`

* refs #18154