Describe Soft And Hard Forks

Based on a suggestion made by @petertodd to the -devel mailing list and
the discussion in that thread by him and other participants.

* We've been using the term "consensus", but this commit introduces a
  formal definition for it and "consensus rules" as part of the block
  chain introduction.

* Describe that consensus rules may change and may happen when they
  do: hard or soft forks.

* Describe how full nodes can detect hard or soft forks, then describe
  how SPV clients can detect hard and soft forks using the more limited
  information available to them.

_includes/references.md

@@ -1,5 +1,4 @@
 [51 percent attack]: /en/developer-guide#term-51-attack "The ability of someone controlling a majority of hashing power to revise transactions history and prevent new transactions from confirming"
-[accidental fork]: /en/developer-guide#term-accidental-fork "When two or more blocks have the same block height, forking the block chain.  Happens occasionally by accident"
 [addresses]: /en/developer-guide#term-address "A 20-byte hash formatted as a P2PKH or P2SH Bitcoin Address"
 [address]: /en/developer-guide#term-address "A 20-byte hash formatted as a P2PKH or P2SH Bitcoin Address"
 [base58Check]: /en/developer-reference#term-base58check "The method used in Bitcoin for converting 160-bit hashes into Bitcoin addresses"
@@ -30,6 +29,8 @@
 [confirmed transactions]: /en/developer-guide#term-confirmation "Transactions included in a block currently on the block chain"
 [confirmation]: /en/developer-guide#term-confirmation "The number of blocks which would need to be modified to remove or modify a transaction"
 [confirmations]: /en/developer-guide#term-confirmation "The number of blocks which would need to be modified to remove or modify a transaction"
+[consensus]: /en/developer-guide#term-consensus "When several nodes (usually most nodes on the network) all have the same blocks in their locally-validated block chain."
+[consensus rules]: /en/developer-guide#term-consensus-rules "The block validation rules that full nodes follow to stay in consensus with other nodes."
 [denomination]: /en/developer-guide#term-denomination "bitcoins (BTC), bitcents (cBTC), millibits (mBTC), microbits (uBTC), or satoshis"
 [difficulty]: /en/developer-guide#term-difficulty "A number corresponding to the target threshold which indicates how difficult it will be to find the next block"
 [double spend]: /en/developer-guide#term-double-spend "Attempting to spend the same satoshis which were spent in a previous transaction"
@@ -38,7 +39,9 @@
 [extended public key]: /en/developer-guide#term-extended-public-key "A public key extended with the chain code so that it can derive child public keys"
 [escrow contract]: /en/developer-guide#term-escrow-contract "A contract in which the spender and receiver store satoshis in a multisig output until both parties agree to release the satoshis"
 [fiat]: /en/developer-guide#term-fiat "National currencies such as the dollar or euro"
+[fork]: /en/developer-guide#term-fork "When two or more blocks have the same block height, forking the block chain."
 [genesis block]: /en/developer-guide#term-genesis-block "The first block created; also called block 0"
+[hard fork]: /en/developer-guide#term-hard-fork "A permanent divergence in the the block chain, commonly occurs when non-upgraded nodes can't validate blocks created by upgraded nodes following newer consensus rules."
 [hardened extended private key]: /en/developer-guide#term-hardened-extended-private-key "A private key whose corresponding public key cannot derive child keys"
 [HD protocol]: /en/developer-guide#term-hd-protocol "The Hierarchical Deterministic (HD) key creation and transfer protocol"
 [header nonce]: /en/developer-reference#term-header-nonce "Four bytes of arbitrary data in a block header used to let miners create headers with different hashes for proof of work"
@@ -132,6 +135,7 @@
 [signature]: /en/developer-guide#term-signature "The result of combining a private key and some data in an ECDSA signature operation which allows anyone with the corresponding public key to verify the signature"
 [signature hash]: /en/developer-guide#term-signature-hash "A byte appended onto signatures generated in Bitcoin which allows the signer to specify what data was signed, allowing modification of the unsigned data"
 [signature script]: /en/developer-guide#term-signature-script "Data generated by a spender which is almost always used as variables to satisfy a pubkey script"
+[soft fork]: /en/developer-guide#term-soft-fork "A temporary fork in the block chain which commonly occurs when miners using non-upgraded nodes violate a new consensus rule their nodes don't know about."
 [spv]: /en/developer-guide#simplified-payment-verification-spv "A method for verifying particular transactions were included in blocks without downloading the entire contents of the block chain"
 [ssl signature]: /en/developer-examples#term-ssl-signature "Signatures created and recognized by major SSL implementations such as OpenSSL"
 [stack]: /en/developer-guide#term-stack "An evaluation stack used in Bitcoin's script language"
@@ -160,9 +164,13 @@
 [X509Certificates]: /en/developer-examples#term-x509certificates
 
 [BFGMiner]: https://github.com/luke-jr/bfgminer
+[BIP16]: https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki
 [BIP21]: https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki
+[BIP30]: https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki
 [BIP32]: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
+[BIP34]: https://github.com/bitcoin/bips/blob/master/bip-0034.mediawiki
 [BIP39]: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
+[BIP50]: https://github.com/bitcoin/bips/blob/master/bip-0050.mediawiki
 [BIP70]: https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki
 [BIP71]: https://github.com/bitcoin/bips/blob/master/bip-0071.mediawiki
 [BIP72]: https://github.com/bitcoin/bips/blob/master/bip-0072.mediawiki