Add changelog

Author: minbi

CHANGELOG.md

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+# Change Log - AWS AppSync Codegen
+
+## [Release 0.17.3](https://github.com/awslabs/aws-appsync-codegen/releases/tag/0.17.3)
+
+### New Features
+
+* Adds support for AWS AppSync Defined Scalars such as `AWSTimestamp`.
+
+## [Release 0.17.2](https://github.com/awslabs/aws-appsync-codegen/releases/tag/0.17.2)
+
+### New Features
+
+* Initial release