releases, readme: Add release notes for 2.1 RC1#586

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lilin90 merged 3 commits intopingcap:masterfrom CaitinChen:add-release-notes-21rc1

Aug 25, 2018

+158-1

Commits on Aug 24, 2018

Commits on Aug 25, 2018

releases, readme: add the release notes for 2.1 RC 1
CaitinChen
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