Maintain the release notes of 0.12.2

[Jialin Qiao]

Hi all,

The RELEASE_NOTES.md in rel/0.12 is updated for 0.12.2.

If you fix a bug to rel/0.12 from now on, please remember to update the
release notes.

Thanks,
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Jialin Qiao
School of Software, Tsinghua University

乔嘉林
清华大学 软件学院