Update release note 1.2.0 (#4928)

* update release note v1.2.0

* update README

* Add upgrade guide link to RELEASE_NOTE

* Old framework retry history cannot be shown after upgrading to v1.2.0

* Add link for all new items

README.md

@@ -6,7 +6,7 @@
 [![Join the chat at https://gitter.im/Microsoft/pai](https://badges.gitter.im/Microsoft/pai.svg)](https://gitter.im/Microsoft/pai?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Version](https://img.shields.io/github/release/Microsoft/pai.svg)](https://github.com/Microsoft/pai/releases/latest)
 
-**OpenPAI [v1.1.1](./RELEASE_NOTE.md#July-2020-version-111) has been released!**
+**OpenPAI [v1.2.0](./RELEASE_NOTE.md#Sep-2020-version-120) has been released!**
 
 With the release of v1.0, OpenPAI is switching to a more robust, more powerful and lightweight architecture. OpenPAI is also becoming more and more modular so that the platform can be easily customized and expanded to suit new needs. OpenPAI also provides many AI user-friendly features, making it easier for end users and administrators to complete daily AI tasks.
 

RELEASE_NOTE.md

@@ -1,6 +1,33 @@
 # OpenPAI Release Note
 
+## Sep 2020 (version 1.2.0)
+
+- Database
+  - New RestServer Arch: RestServer -> DB -> ApiServer (#4651)
+- Webportal
+  - Job list paging in server side (#4651)
+  - Change job createdTime to submissionTime (#4761)
+  - VC && Group experience for Admin (AAD Mode) (#4800)
+  - Support SKU count and SKU type in job submission page (#4796)
+  - Upgrade api/v1 code to api/v2 (#4704)
+  - Show "More Diagnostics" in job detail page (#4670)
+- Marketplace
+  - [Unified PAI protocol](https://github.com/microsoft/openpaimarketplace#49)
+  - [Merge examples](https://github.com/microsoft/openpaimarketplace#41)
+- Others
+  - HiveD improvement (#4868)
+  - Robustness improvement (#4694)
+  - Fix logrotate issue (#4792)
+  - Fix [runtime image check issue](https://github.com/microsoft/openpai-runtime#17)
+  - Fix error message for SKU (#4602)
+  - [Quick start for AKS-Engine](https://github.com/microsoft/pai/blob/v1.2.0/contrib/aks-engine/readme.md)
+- [Old framework retry history cannot be shown after upgrading to v1.2.0](https://github.com/microsoft/pai/issues/4930)
+- [Upgrade Guide](https://openpai.readthedocs.io/en/latest/manual/cluster-admin/upgrade-guide.html)
+
+For more details about this release, please refer to [detailed release note](https://github.com/microsoft/pai/releases/tag/v1.2.0).
+
 ## July 2020 (version 1.1.1)
+
 [minor release with hot fix](https://github.com/microsoft/pai/releases/tag/v1.1.1).
 
 ## July 2020 (version 1.1.0)