add changelog for v0.3 (#59)

Signed-off-by: haoqing0110 <qhao@redhat.com>

Author: haoqing0110

CHANGELOG/CHANGELOG-v0.3.md

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+# Changelog since v0.2.0
+All notable changes to this project will be documented in this file.
+
+## v0.3.0
+
+### New Features
+* Support extensible scheduling. User is able to select clusters based on customized scores. ([#51](https://github.com/open-cluster-management-io/placement/pull/51) [@haoqing0110](https://github.com/haoqing0110))
+
+### Added
+* Add scalability test for 100/1000/2000 managedclusters. ([#41](https://github.com/open-cluster-management-io/placement/pull/41) [@suigh](https://github.com/suigh))
+* Add scalability test for placement create/update. ([#44](https://github.com/open-cluster-management-io/placement/pull/44) [@suigh](https://github.com/suigh))
+* Add scalability test doc. ([#45](https://github.com/open-cluster-management-io/placement/pull/45) [@suigh](https://github.com/suigh))
+* Add resync controller to resync placements using customized scores. ([#55](https://github.com/open-cluster-management-io/placement/pull/55) [@haoqing0110](https://github.com/haoqing0110))
+
+### Changes
+* Update Golang to v1.17. ([#57](https://github.com/open-cluster-management-io/placement/pull/57) [@haoqing0110](https://github.com/haoqing0110))
+
+### Bug Fixes
+* Fix [error reported after creating more than 1000 managedclusters](https://github.com/open-cluster-management-io/placement/issues/40) with adding length check for event. ([#43](https://github.com/open-cluster-management-io/placement/pull/43) [@suigh](https://github.com/suigh)) 
+* Remind user to deploy placement controller before running some test cases. ([#47](https://github.com/open-cluster-management-io/placement/pull/47) [@suigh](https://github.com/suigh)) 
+* Update golang.org/x/crypto to fix CVE-2021-43565. ([#58](https://github.com/open-cluster-management-io/placement/pull/58) [@haoqing0110](https://github.com/haoqing0110))
+
+### Removed & Deprecated
+N/C

CHANGELOG/README.md

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+# CHANGELOGs
+
+- [CHANGELOG-v0.3.md](./CHANGELOG-v0.3.md)
+- [CHANGELOG-v0.2.md](./CHANGELOG-v0.2.md)
+- [CHANGELOG-v0.1.md](./CHANGELOG-v0.1.md)