Ensure Sparkle DSA signing procedure has random seed with high entropy

[riastradh-brave]

• Write down the procedure that we use to make Sparkle DSA signatures on apps.
• Write down a SCARY WARNING that it must not be done in a VM snapshot whose state might be rolled back.
• Ensure that whatever procedure we use -- generate_appcast, openssl dgst -dss1 -sign -- uses a reasonable procedure seeded with high entropy to generate the per-signature secret, and/or uses the equivalent of RFC 6979.

If the procedure of making a signature has low entropy and RFC 6979 or equivalent is not used, then the signatures can leak the private key, as, for example, Sony discovered the hard way with the PlayStation 3 firmware update signing key.