ZIP 203: changes for Blossom and NU5.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

README.template

@@ -46,6 +46,7 @@ This is the list of ZIPs relevant to the proposed NU5 Upgrade, which
 is planned to activate on Mainnet at the beginning of October 2021:
 
 - `ZIP 32: Shielded Hierarchical Deterministic Wallets <zip-0032.rst>`__ (updated)
+- `ZIP 203: Transaction Expiry <zip-0203.rst>`__ (updated)
 - `ZIP 209: Prohibit Negative Shielded Chain Value Pool Balances <zip-0209.rst>`__ (updated)
 - `ZIP 212: Allow Recipient to Derive Ephemeral Secret from Note Plaintext <zip-0212.rst>`__ (updated)
 - `ZIP 213: Shielded Coinbase <zip-0213.rst>`__ (updated)

zip-0203.rst

@@ -49,7 +49,8 @@ after which transactions will be removed from the mempool if they have not been
 The data type for ``nExpiryHeight`` will be ``uint32_t``. If used in combination with
 ``nLockTime``, both ``nLockTime`` and ``nExpiryHeight`` must be block heights.
 ``nExpiryHeight`` will never be a UNIX timestamp, unlike ``nLockTime`` values, and thus
-the maximum expiry height will be 499999999.
+the maximum expiry height will be 499999999 (but see the exception for coinbase
+transactions described in `Changes for NU5`_).
 
 For the example below, the last block that the transaction below could possibly be
 included in is 3539. After that, it will be removed from the mempool.
@@ -77,6 +78,22 @@ convention.
 Every time a transaction expires and should be removed from the mempool, so should all
 of its dependent transactions.
 
+Changes for Blossom
+-------------------
+
+On Blossom activation [#zip-0206]_, the default changes to 40 blocks from the current
+height, which still represents about 50 minutes at the revised 75-second target block
+spacing.
+
+Changes for NU5
+---------------
+
+As mentioned above, ``nExpiryHeight`` is ignored for coinbase transactions. However, from
+NU5 activation [#zip-0252]_, the ``nExpiryHeight`` field of a coinbase transaction MUST
+be set equal to the block height. Also, for coinbase transactions only, the bound of
+499999999 on ``nExpiryHeight`` is removed. The motivation is to ensure that transaction
+IDs remain unique, as explained in more detail in a note in Section 7.1 of the protocol
+specification [#protocol-txnencodingandconsensus]_.
 
 Wallet behavior and UI
 ----------------------
@@ -126,3 +143,6 @@ References
 ==========
 
 .. [#zip-0201] `ZIP 201: Network Peer Management for Overwinter <zip-0201.rst>`_
+.. [#zip-0206] `ZIP 206: Deployment of the Blossom Network Upgrade <zip-0206.rst>`_
+.. [#zip-0252] `ZIP 252: Deployment of the NU5 Network Upgrade <zip-0252.rst>`_
+.. [#protocol-txnencodingandconsensus] `Zcash Protocol Specification, Version 2020.2.6. Section 7.1: Transaction Encoding and Consensus <protocol/protocol.pdf#txnencodingandconsensus>`_