Please document: signconf file format and ods-signer commands

Please document the interface between ods-enforcer and ods-signer.  The current situation leaves the following to be desired:

  * the commands sent to ods-signer are listed, but not explained

  * I found no documentation of the .signconf file format and how it is processed

This situation probably exists under the assumption that users will always use the combination of ods-signer and ods-enforcer, which I would argue is an unnecessary reduction of the freedom of our users.  We could explain in some detail why it could make sense to use ods-signer on its own (and are indeed working on a replacement for the ods-enforcer ourselves).