update changelog for ubuntu 18.04 build
### Bug Fixes
- [[#1600]](https://github.com/Azure/azure-sdk-for-c/pull/1600) Make sure `az_json_writer_append_json_text()` appends a comma between elements of a JSON array.
+- [[#1580]](https://github.com/Azure/azure-sdk-for-c/pull/1580) Fix build on Ubuntu 18.04 by updating CMake policy and MSVC runtime libraries.
## 1.1.0-beta.2 (2020-11-11)
