Feedback on signatures.md

I understand we have yet to update this portion of the spec, but I'll write out forward thinking feedback.

1. Would be good to explain why we are using a specific type of signature. For example, for BLS, we could say something like.

> Signature aggregation reduces the total storage load of each message on the blockchain. This allows more messages to be included per block. And in turn, increases the total storage available to Filecoin as well as make individual transactions cheaper.

2. Would be good to list references to other protocols where the signatures were implemented for reference / credibility. For example, for BLS we could discuss Zcash or Chia and list some links:
- https://twitter.com/bramcohen
- https://github.com/Chia-Network/bls-signatures
- https://z.cash/blog/new-snark-curve/
- https://ethresear.ch/t/pragmatic-signature-aggregation-with-bls/2105

3. Let's avoid references to slack, and instead copy in the relevant discussion into Github somewhere. Slack messages will disappear every 2 months (I think?) and are also private to PL. Specifically referencing the last sentence:
> Discussion on slack: https://protocollabs.slack.com/archives/G7XUR2TU2/p1528984460000977

4. Should we write-up any context that allows integration partners to use less familiar signatures like BLS? I'm thinking of exchanges / wallets that need to make adjustments on their end to support BLS.

Start by reading signatures.md, focusing on the BLS discussion and the sentence linking to Slack. Review the four requested improvements: explain signature choices, add protocol references, preserve relevant discussion outside Slack, and document integration context for less familiar signatures. Done means the requested context and references are incorporated into the specification.

Written by the indexing model from the issue text.