How to version the specification and what version of what runtime are supported

[seebees]

Now that we have a specification,
as this will change how is this change tracked.
Also, what versions do individual ESDK support
and how is this tested and maintained?

[mattsb42-aws]

note: we are currently versioning each spec feature doc independently. I think that for most features this is the correct way to go, because most of them are independent of each other (ex: the AWS KMS keyring doesn't really have anything to do with the message format).

Most of the current docs are versioned as 0.1.0-preview, but we do not define a versioning policy.

I propose that we:

1. Determine what 1.0.0 means for the spec and features.
2. Once we hit 1.0.0, follow Semantic Versioning.
3. While we are in 0.Y.Z, allow breaking changes in Y increments.

[mattsb42-aws]

Another thing to call out in this issue, as discussed here[1]: where/how should we record which compatibility matrices between spec documents and implementations?

[1] #105 (comment)