#[52257091] Adding initial changelog

Sidekick: @nathanhumbert & @rantler 
Review: @leeatchison 

Adding initial markdown changelog. Discussed with Nathan that we would leave off the Changes and Bug Fixes sections until we have changes and bug fixes to report. Otherwise the changelog could grow long with empty sections that would lead to clutter.

Moving forward we should update the vNext section with new features, breaking changes, and bug fixes as we make changes to the SDK. The version and date will be filled in prior to release and tagging.