Add Changelog (#586)

Author: jamiebrynes7

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Added
+
+- Added support for the Steam authentication flow.
+- Added the ability to acknowledge `AuthorityLossImminent` messages.
+- Added an `Open Inspector` button to the `SpatialOS` menu in the Unity Editor.
+- Added support for local mobile development.
+- Added a changelog.
+- Added field level dirty markers in components. This allows for partial automatic component updates to be sent.
+
+### Changed
+
+- Changed the allocation type used internally for Unity ECS chunk iteration from `Temp` to `TempJob`
+- Running a build in the Editor no longer automatically selects all scenes in the Unity build configuration
+- `Improbable.Gdk.Core.Snapshot.AddEntity` now returns the `EntityId` assigned in the snapshot.
+
+### Fixed
+
+- Fixed a bug where deserialising multiple events in a single component update only returned N copies of the last event received, where N is the number of events in the update.
+- Fixed a broken link to the setup guide in an error message.
+
+## `0.1.1` - 2018-10-19
+
+### Added
+
+- Better error messages when missing build support for a target platform.
+- Better error messages for common problems when downloading the Worker SDK.
+
+### Changed
+
+- Position updates are now sent after all other updates.
+- Simplified the heartbeating system in the `PlayerLifecycle` feature module.
+- Updated the `README` and "Get Started" guide.
+
+### Fixed
+
+- The `GameLogic` worker is run in headless mode.
+- The `Clean All Workers` menu item now works.
+
+## `0.1.0` - 2018-10-10
+
+The initial alpha release of the SpatialOS GDK for Unity.