Merge branch 'ps/document-breaking-changes'

The structure of the document that records longer-term project decisions to deprecate/remove/update various behaviour has been outlined. * ps/document-breaking-changes: BreakingChanges: document that we do not plan to deprecate git-checkout BreakingChanges: document removal of grafting BreakingChanges: document upcoming change from "sha1" to "sha256" docs: introduce document to announce breaking changes