[0.12] Update release notes

doc/release-notes.md

@@ -35,7 +35,108 @@ earlier.
 Notable changes
 ===============
 
-The p2p alert system is off by default. To turn on, use `-alert` with startup configuration.
+First version bits BIP9 softfork deployment
+-------------------------------------------
+
+This release includes a soft fork deployment to enforce [BIP68][],
+[BIP112][] and [BIP113][] using the [BIP9][] deployment mechanism.
+
+The deployment sets the block version number to 0x20000001 between
+midnight 1st May 2016 and midnight 1st May 2017 to signal readiness for 
+deployment. The version number consists of 0x20000000 to indicate version
+bits together with setting bit 0 to indicate support for this combined
+deployment, shown as "csv" in the `getblockchaininfo` RPC call.
+
+For more information about the soft forking change, please see
+<https://github.com/bitcoin/bitcoin/pull/7648>
+
+This specific backport pull-request can be viewed at
+<https://github.com/bitcoin/bitcoin/pull/7543>
+
+[BIP9]: https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki
+[BIP68]: https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki
+[BIP112]: https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki
+
+BIP68 soft fork to enforce sequence locks for relative locktime
+---------------------------------------------------------------
+
+[BIP68][] introduces relative lock-time consensus-enforced semantics of
+the sequence number field to enable a signed transaction input to remain
+invalid for a defined period of time after confirmation of its corresponding
+outpoint.
+
+For more information about the implementation, see
+<https://github.com/bitcoin/bitcoin/pull/7184>
+
+BIP112 soft fork to enforce OP_CHECKSEQUENCEVERIFY
+--------------------------------------------------
+
+[BIP112][] redefines the existing OP_NOP3 as OP_CHECKSEQUENCEVERIFY (CSV)
+for a new opcode in the Bitcoin scripting system that in combination with
+[BIP68][] allows execution pathways of a script to be restricted based
+on the age of the output being spent.
+
+For more information about the implementation, see
+<https://github.com/bitcoin/bitcoin/pull/7524>
+
+BIP113 locktime enforcement soft fork
+-------------------------------------
+
+Bitcoin Core 0.11.2 previously introduced mempool-only locktime
+enforcement using GetMedianTimePast(). This release seeks to
+consensus enforce the rule.
+
+Bitcoin transactions currently may specify a locktime indicating when
+they may be added to a valid block.  Current consensus rules require
+that blocks have a block header time greater than the locktime specified
+in any transaction in that block.
+
+Miners get to choose what time they use for their header time, with the
+consensus rule being that no node will accept a block whose time is more
+than two hours in the future.  This creates a incentive for miners to
+set their header times to future values in order to include locktimed
+transactions which weren't supposed to be included for up to two more
+hours.
+
+The consensus rules also specify that valid blocks may have a header
+time greater than that of the median of the 11 previous blocks.  This
+GetMedianTimePast() time has a key feature we generally associate with
+time: it can't go backwards.
+
+[BIP113][] specifies a soft fork enforced in this release that
+weakens this perverse incentive for individual miners to use a future
+time by requiring that valid blocks have a computed GetMedianTimePast()
+greater than the locktime specified in any transaction in that block.
+
+Mempool inclusion rules currently require transactions to be valid for
+immediate inclusion in a block in order to be accepted into the mempool.
+This release begins applying the BIP113 rule to received transactions,
+so transaction whose time is greater than the GetMedianTimePast() will
+no longer be accepted into the mempool.
+
+**Implication for miners:** you will begin rejecting transactions that
+would not be valid under BIP113, which will prevent you from producing
+invalid blocks when BIP113 is enforced on the network. Any
+transactions which are valid under the current rules but not yet valid
+under the BIP113 rules will either be mined by other miners or delayed
+until they are valid under BIP113. Note, however, that time-based
+locktime transactions are more or less unseen on the network currently.
+
+**Implication for users:** GetMedianTimePast() always trails behind the
+current time, so a transaction locktime set to the present time will be
+rejected by nodes running this release until the median time moves
+forward. To compensate, subtract one hour (3,600 seconds) from your
+locktimes to allow those transactions to be included in mempools at
+approximately the expected time.
+
+For more information about the implementation, see
+<https://github.com/bitcoin/bitcoin/pull/6566>
+
+Miscellaneous
+-------------
+
+The p2p alert system is off by default. To turn on, use `-alert` with
+startup configuration.
 
 0.12.1 Change log
 =================