Document consensus rules from Zcash spec: 7.10 Payment of Funding Streams

[mpguerra]

For each consensus rule:

• check that the rule is implemented in Zebra
• make sure the latest version of the rule is quoted in the docs for the code that implements it
• make sure the code implements the latest version of the rule

Consensus rule

https://zips.z.cash/protocol/protocol.pdf#fundingstreams

Implementation

Implemented in:

• Calc Block Subsidy and Funding Streams #338 (PRs: Validate funding stream amounts in coinbase transaction #3017, Validate funding stream addresses #3040)