How do miners signal upgrade readiness? Other things?

One consideration for a smooth upgrade process in the filecoin network is getting a read for the network's willingness and preparation for a protocol change. There is a precedent to allow miners to write arbitrary data somewhere in the block (i.e. BTC's coinbase input). This way miners can signal their readiness to upgrade. This isn't strictly necessary as everyone can vote by hard fork but it could make things easier.

Right now we don't have something like this in our [block format](https://github.com/filecoin-project/specs/blob/master/data-structures.md#block). Should we?

Start with the block format section in data-structures.md and review how the current Filecoin block represents miner data. Determine whether upgrade-readiness signaling belongs in the format, then document a decided proposal and its impact on the protocol specification.

Written by the indexing model from the issue text.