doc: create 0.1.21 changelog

changelogs/0.1.21.md

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+Date: 20-08-2022
+
+Tag: 0.1.21
+
+## Compatibility
+
+VShard 0.1.21 is fully compatible with the previous VShard versions.
+
+## Bugs fixed
+
+* Fixed a bug that the rebalancing didn't work if the sharded spaces had
+  unusual types like 'varbinary' (gh-327, gh-328).
+
+* A pack of fixes for problems with accessing buckets on replicas. They were not
+  properly protected against rebalancer and users could read too outdated data
+  or even inconsistent data (gh-173).
+
+* Fixed a bug that if vshard's connection netbox fibers were killed on router or
+  storage, the connection could be not re-established on some Tarantool versions
+  making the peer inaccessible from the given node at all (gh-341).