Document breaking SDK changes vs breaking behavioral changes

[mattjohnsonpint]

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With "Semantic Versioning", it's well understood that a major version change (i.e. 3.x.x to 4.x.x) would be considered a possible "breaking" change. This concept applies well for SDKs.

However, sometimes we need to make non-breaking API changes, that change behavior with regard to how data comes through to Sentry. For example, something that changes grouping, or "fixes" data bugs that would re-open a previously-ignored alert.

We should think through how to communicate these types of changes better, across all our SDKs. There should be no surprise side-effects of upgrading to the latest SDK.

[mattjohnsonpint]

@simoncropp - want to take a first stab at a draft?