Add Changelog (#57)

Added changelog and backfilled with previous release notes and merged PRs since.

Author: jamiebrynes7

CHANGELOG.md

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+# Changelog
+
+## Unreleased
+
+### Fixed
+
+- Fixed an issue where newer versions of Unity would fail to open the project with errors like: `ParticleSystem` not found.
+
+### Removed
+
+- Removed `GroundChecker` and replaced with a cheaper ground checking functionality in the base `CharacterControllerMotor` class.
+- Removed unused code and resources related to the "desert" in map generation.
+
+## `0.1.1` - 2018-10-19
+
+### Added
+
+- Added external worker configurations for MacOS for all server workers in the project.
+
+### Changed
+
+- Updated to GDK for Unity version `0.1.1`.
+- Improved Simulated Players; they fall off the navmesh less often.
+
+### Fixed
+
+- Fixed a bug that caused spawn points to be in the air, walls, or floor.
+- Fixed a cross-worker spawning issue that resulted in players not being able to move and float in the air.
+
+## `0.1.0` - 2018-10-10
+
+The initial alpha release of the SpatialOS GDK for Unity FPS Starter Project.