Add links to BIPs and Stratum info

[Sjors]

[Sjors]

Unrelated to this PR, but it might be useful to remind readers of the current block serialization format before introducing the new one. In particular remind people that the block header is immediately followed by the transaction count which is immediately followed by an array of transactions, the first of which is the coinbase transaction.

If I understand correctly this BIP takes advantage of that serialization format for backwards compatibility, by tricking older nodes into thinking the transaction array is empty. Although I noticed you proposed some changes after writing this.