add change log (tikv#1024)

CHANGELOG.md

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+# PD Change Log
+## v2.0.0-rc.5
+### New Features
+* Support adding the learner node
+### Improves
+* Optimize the Balance Region Scheduler to reduce scheduling overhead
+* Adjust the default value of `schedule-limit` configuration
+* Fix the compatibility issue when adding a new scheduler
+### Bug Fixes
+* Fix the issue of allocating IDs frequently
+
+## v2.0.0-rc.4
+### New Features
+* Support splitting Region manually to handle the hot spot in a single Region
+### Improves
+* Optimize metrics
+### Bug Fixes
+* Fix the issue that the label property is not displayed when `pdctl` runs `config show all`
+
+## v2.0.0-rc3
+### New Features
+* Support Region Merge, to merge empty Regions or small Regions after deleting data
+### Improves
+* Ignore the nodes that have a lot of pending peers during adding replicas, to improve the speed of restoring replicas or making nodes offline
+* Optimize the scheduling speed of leader balance in scenarios of unbalanced resources within different labels
+* Add more statistics about abnormal Regions
+### Bug Fixes
+* Fix the frequent scheduling issue caused by a large number of empty Regions