Define meaning / purpose of version numbering in shape tree specification

[barath]

Version numbering in the shape tree specification may enable easy distinguishing of new vs. old instances. However the meaning of the version number should be made explicit.

Pros of version numbering:

• Enables easy comparison of definitions
• Enables easy checking of temporal ordering of definitions

Cons of version numbering:

• Downstream uses of shape trees and shapes are unknown and thus it is difficult to know what constitutes a breaking change that would be a minor vs. major version number.
• Version numbering requires synchronization between the data structure and the version number; it's possible that the version number will be accidentally left the same during an update, creating a risk of reliance upon it.

Proposed resolution: a monotonically-increasing version number (with no distinction between major/minor) is simple and effective without overclaiming its meaning.

cc: @elf-pavlik @justinwb