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.

_autocrossref.yaml

@@ -48,6 +48,8 @@ confirmed:
 confirmation:
 confirmations:
 confirmed transactions:
+consensus:
+consensus rules:
 denomination:
 denominations: denomination
 DER format: der
@@ -65,8 +67,11 @@ extended keys: extended key
 extended private key:
 extended public key:
 fiat:
-fork: accidental fork
+fork:
+forks: fork
 genesis block:
+hard fork:
+hard forks: hard fork
 hardened extended private key:
 HD protocol:
 header nonce:
@@ -184,6 +189,8 @@ signature hash:
 signature script:
 signature scripts: signature script
 signatures: signature
+soft fork:
+soft forks: soft fork
 SPV:
 stack:
 stale block:
@@ -217,10 +224,14 @@ wallet import format:
 x.509: x509
 X509Certificates:
 
-## BIPS in numerical order; don't use padding zeros (e.g. BIP70 not BIP0070)
+## BIPs in numerical order; don't use padding zeros (e.g. BIP70 not BIP0070)
+BIP16:
 BIP21:
+BIP30:
 BIP32:
+BIP34:
 BIP39:
+BIP50:
 BIP70:
 BIP71:
 BIP72: