Somewhat stabilize repository metadata format about meta

[lberrymage]

This is now implemented in accrescent/accrescent@c317c28. All that's left to do is the following:

• Update doc to reflect changes in how unsigned repodata is handled
• PDF-ify and host doc on Accrescent website
• Get doc reviewed by someone knowledgeable in cryptography

[lberrymage]

I'm closing this issue with the argument that the repodata format has been "somewhat stabilized." It would still be good to have Accrescent's repodata verification and other inner workings reviewed before the public beta. However, the project is rapidly changing at the moment and it wouldn't make sense to have current processes/code undergo extensive review when they may change significantly in the near future as new needs are discovered and addressed.