Add a Revision History to the Spec

[mobilinkd]

Please add a Protocol Revision History which lists substantive changes, especially those that would break backwards compatibility with existing implementations. I have not followed changes to the spec for a bit and would like to update my implementations. But it is not clear what part of the OTA protocol has changed and when. The git commit history is valuable, but is no substitute for a concise revision history as the git history contains a lot of noise from reformatting and clarifications that don't substantively modify the protocol. I am not necessarily suggesting that we go back and document prior changes, just that any new changes are documented in a Revision History section.

Please also add a version number to the spec using semantic versioning, and tag those versions in Git. This means breaking changes (any change that could cause devices in the field to no longer interoperate) will have the major version number increased, any extensions result in the minor version being incremented, and any reformatting or clarification receives a patch-level change.

Changes that break backwards compatibility in a meaningful way should be very carefully considered now that commercial products implementing M17 exist. Even substantive extensions to M17 may result in future incompatibilities in the field.