Create release announcement for v1.0.0

[qiujian16]

release note to read from these repositories:

• api: https://github.com/open-cluster-management-io/api/releases/tag/v1.0.0
• ocm: https://github.com/open-cluster-management-io/ocm/releases/tag/v1.0.0
• addon-framework: https://github.com/open-cluster-management-io/addon-framework/releases/tag/v1.0.0
• clusteradm: https://github.com/open-cluster-management-io/clusteradm/releases/tag/v1.0.0

[mikeshng]

CC @augustrh Should we do something special for the website instead of just a regular release note like the prior releases?