Update CHANGELOG.md to include User-Agent Header addition and versioning for v1.5.1-preview

Author: YunchuWang

CHANGELOG.md

@@ -1,4 +1,6 @@
 ## placeholder
+* Add User-Agent Header to gRPC Metadata ([#213](https://github.com/microsoft/durabletask-java/pull/213))
+## ## v1.5.1-preview
 * DTS Support ([#201](https://github.com/microsoft/durabletask-java/pull/201))
 * Add automatic proto file download and commit hash tracking during build ([#207](https://github.com/microsoft/durabletask-java/pull/207))
 * Fix infinite loop when use continueasnew after wait external event ([#183](https://github.com/microsoft/durabletask-java/pull/183))