[noise] Revision 22 - editorial

[Trevor Perrin]

Tom Ritter gave some great editorial feedback, so I tried to clarify things:

https://github.com/trevp/noise/blob/master/noise.md

 * Better explanations of patterns in intro

 * New section (6) explaining prologue

 * Broke out some of the text in pseudocode-like bullet points (for
Split() and InitializeSymmetric())

 * Added a security consideration about GCM data volumes


Trevor