Document consensus rules from Zcash spec 7.1.2: Transaction Size

[mpguerra]

• [Pre-Sapling] The encoded size of the transaction MUST be less than or equal to 100000 bytes.
• [NU5 onward] In a version 5 transaction, the reserved bits 2..7 of the flagsOrchard field MUST
  be zero.
• [NU5 onward] nSpendsSapling, nOutputsSapling, and nActionsOrchard MUST all be less than 2^16.
• [Pre-Sapling] If effectiveVersion = 1 or nJoinSplit = 0, then both tx_in_count and tx_out_count MUST be nonzero.
• [Sapling onward] If effectiveVersion < 5, then at least one of tx_in_count, nSpendsSapling, and nJoinSplit MUST be nonzero.
• [Sapling onward] If effectiveVersion < 5, then at least one of tx_out_count, nOutputsSapling, and nJoinSplit MUST be nonzero.
• [NU5 onward] If effectiveVersion >= 5 then this condition MUST hold: tx_in_count > 0 or nSpendsSapling > 0 or (nActionsOrchard > 0 and enableSpendsOrchard = 1).
• [NU5 onward] If effectiveVersion >= 5 then this condition MUST hold: tx_out_count > 0 or nOutputsSapling > 0 or (nActionsOrchard > 0 and enableOutputsOrchard = 1).
• [NU5 onward] If effectiveVersion >= 5 and nActionsOrchard > 0, then at least one of enableSpendsOrchard and enableOutputsOrchard MUST be 1.
• [Sapling onward] If effectiveVersion = 4 and there are no Spend descriptions or Output descriptions, then valueBalanceSapling MUST be 0.