add option enable guest invitation

Author: christophdb

docs/configuration/available-variables.md

@@ -166,6 +166,7 @@ The webinterface of seafile...
 | `SEAHUB__EMAIL_PORT`                                 | Port used for connecting to the SMTP server.                                                                                                                                                                                                                                            | "25"                                                                                                                                                                                                                           |
 | `SEAHUB__DEFAULT_FROM_EMAIL`                         | Default from email address, typically set to the email host user.                                                                                                                                                                                                                       | "`SEAHUB__EMAIL_HOST_USER`"                                                                                                                                                                                                    |
 | `SEAHUB__SERVER_EMAIL`                               | Email address used for server-originated emails, typically set to the email host user.                                                                                                                                                                                                  | "`SEAHUB__EMAIL_HOST_USER`"                                                                                                                                                                                                    |
+| `SEAHUB__ENABLE_GUEST_INVITATION`                    | Allow users to invite other uses by their email.                                                                                                                                                                                                                                        | "False"                                                                                                                                                                                                                        |
 
 ## Seafdav