Add change log file

CHANGELOG.md

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+# Change Log
+All notable changes to the On Demand Service Broker will be documented here
+
+## Unreleased
+Work in progress appears here. On release, create a section below with version number and date
+and move the unreleased items to it.
+
+### Added
+- As you finish a logical group of stories adding new functionality
+- Insert a brief description here
+
+### Changed
+- Detail any modified functionality
+- here
+
+### Removed
+- And if any functionality has been removed
+- Mention it in a line here
+
+## [0.21.2] - 2018-05-10
+### Added
+
+### Changed
+
+### Removed