Document spec deviations with rationale

[hdost]

While I'm not sure if for this specific case we want to, but for the purposes of posterity and avoid needing to do archeological digging every time a decision is made it might be worth having a doc for this.
Known deviation mentioned:

• SDk/Sampler concepts leaked in API #1043

[hdost]

The parentheses in this comment sounds like another deviation. #1293 (comment)

[cijothomas]

From SIG call:
We already has some known deviations, so lets start with documenting that as an md. We can start with just linking to this issue, which can be used to make the md file