Document consensus rules from Zcash spec 7.1.2: Transaction Header

[mpguerra]

• The transaction version number MUST be greater than or equal to 1.
• [Pre-Overwinter] The fOverwintered fag MUST NOT be set.
• [Overwinter onward] The fOverwintered flag MUST be set.
• [Overwinter onward] The version group ID MUST be recognized.
• [Overwinter only, pre-Sapling] The transaction version number MUST be 3, and the version group ID MUST be 0x03C48270.
• [Sapling to Canopy inclusive, pre-NU5] The transaction version number MUST be 4, and the version group ID MUST be 0x892F2085.
• [NU5 onward] The transaction version number MUST be 4 or 5. If the transaction version number is 4 then the version group ID MUST be 0x892F2085. If the transaction version number is 5 then the version group ID MUST be 0x26A7270A.
• [NU5 onward] The nConsensusBranchId field MUST match the consensus branch ID used for SIGHASH transaction hashes, as specified in [ZIP-244].
• [Overwinter to Canopy inclusive, pre-NU5] nExpiryHeight MUST be less than or equal to 499999999.
• [NU5 onward] nExpiryHeight MUST be less than or equal to 499999999 for non-coinbase transactions.
• [Overwinter onward] If a transaction is not a coinbase transaction and its nExpiryHeight field is nonzero, then it MUST NOT be mined at a block height greater than its nExpiryHeight.