Update README.md

add security note regarding open registration

vaultwarden/README.md

@@ -2,3 +2,7 @@
 
 - https://github.com/dani-garcia/vaultwarden
 - https://github.com/Bruceforce/vaultwarden-backup
+
+# Notes
+
+Disable open user registration after setting up your account. You can find the option as environment variable in the compose file.