Add note on clearance procedure for devices already registered in the DDB

[kerel-fs]

When trying to register a second-hand device where the previous owner didn't remove the registration in the DDB, the user is presented with the following:

As was described by @snip on the ML, the user should send a mail to contact@glidernet.org with details in this case (after trying to reach out to the previous owner).

This should be communicated in the dialog above (or a link to a description should be included).

[Scumi]

I don't get any reply on the e-mail address provided. What is the current method of re-registering devices or solving other issues like lost accounts? Not sure where else to ask as - you guessed it - the contact mail appears to be not getting checked.