CHANGELOG: add changelog for v2.1.2 (#8767)

CHANGELOG.md

@@ -1,6 +1,16 @@
 # TiDB Changelog
 All notable changes to this project will be documented in this file. See also [Release Notes](https://github.com/pingcap/docs/blob/master/releases/rn.md), [TiKV Changelog](https://github.com/tikv/tikv/blob/master/CHANGELOG.md) and [PD Changelog](https://github.com/pingcap/pd/blob/master/CHANGELOG.md).
 
+## [2.1.2] 2018-12-21
+* Make TiDB compatible with TiDB-Binlog of the Kafka version [#8747](https://github.com/pingcap/tidb/pull/8747)
+* Improve the exit mechanism of TiDB in a rolling update [#8707](https://github.com/pingcap/tidb/pull/8707)
+* Fix the panic issue caused by adding the index for the generated column in some cases [#8676](https://github.com/pingcap/tidb/pull/8676)
+* Fix the issue that the optimizer cannot find the optimal query plan when `TIDB_SMJ Hint` exists in the SQL statement in some cases [#8729](https://github.com/pingcap/tidb/pull/8729)
+* Fix the issue that `AntiSemiJoin` returns an incorrect result in some cases [#8730](https://github.com/pingcap/tidb/pull/8730)
+* Improve the valid character check of the `utf8` character set [#8754](https://github.com/pingcap/tidb/pull/8754)
+* Fix the issue that the field of the time type might return an incorrect result when the write operation is performed before the read operation in a transaction [#8746](https://github.com/pingcap/tidb/pull/8746)
+
+
 ## [2.1.1] 2018-12-12
 
 ### SQL Optimizer/Executor