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.

img/dev/en-soft-fork.dot

@@ -0,0 +1,47 @@
+digraph {
+
+size=6.25;
+rankdir=LR
+//splines = ortho;
+ranksep = 0.2;
+nodesep = 0.1;
+
+edge [ penwidth = 1.75, fontname="Sans" ]
+node [ penwidth = 1.75, shape = "box", fontname="Sans", label = "", width=0.3, height=0.3 ]
+graph [ penwidth = 1.75, fontname="Sans" ]
+
+invis1 [ shape = "none", label = "Blocks\nFrom\nUpgraded\nNodes" ]
+invis0 [ shape = "none", label = "Blocks\nFrom Non-\nUpgraded\nNodes" ];
+
+subgraph cluster_honest {
+block0 [ label = "Follows\nOld\nRules" ];
+block1 [ label = "Follows\nOld\nRules" ];
+block2_1 [ label = "Follows Old Rules\nBut Violates\nNew Rules", style = "filled" ];
+
+block2_1 -> block4 [ style = "invis", minlen = 2 ];
+
+block4 [ label = "Follows\nOld\nRules" ];
+
+style = "invis";
+}
+
+
+subgraph cluster_attack {
+block2 [ label = "Follows\nOld & New\nRules" ];
+block3 [ label = "Follows\nOld & New\nRules" ];
+
+style = "invis"
+}
+
+
+
+invis0 -> block0 [ minlen = 2, style = "dashed" ];
+
+block0 -> block1 -> block2 -> block3 -> block4;
+
+block1 -> block2_1
+
+
+
+label = "A Soft Fork:  Blocks Violating New Rules Are Made Stale By The Upgraded Mining Majority"
+}