doc: add changelog-2.1.md

Author: Cyber-SiKu

CHANGELOG-2.1.md

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+# CHANGELOG of v2.1
+
+## new features
+
+- [CurveFS: a curve filesystem can be mounted by multi fuse clients.](https://github.com/opencurve/curve/pull/1101)
+
+- [CurveFS: support summary info in dir xattr.](https://github.com/opencurve/curve/pull/1150)
+
+- [CurveFS: support Multiple s3.](https://github.com/opencurve/curve/pull/1132)
+
+## optimization
+
+- [CurveFS client: adapter lru list for disk cache.](https://github.com/opencurve/curve/pull/1088)
+
+- [CurveFS: meta balance.](https://github.com/opencurve/curve/pull/1105)
+
+- [CurveFS metaserver: let s3 compact task store a shared copyset node.](https://github.com/opencurve/curve/pull/1165)
+
+- [CurveFS: add metric for mds topology to update metaserver metric.](https://github.com/opencurve/curve/pull/1177)
+
+## bug fix
+
+- [CurveFS client: fix bug of getleader always fails causes stack overflow.](https://github.com/opencurve/curve/pull/1070)
+
+- [CurveFS tool: fix copyset health check error.](https://github.com/opencurve/curve/pull/1024)
+
+- [CurveFS client: fix client release read data cache core dump.](https://github.com/opencurve/curve/pull/1090)
+
+- [CurveBS nbd: fix misspell in log.](https://github.com/opencurve/curve/pull/1073)
+
+- [CurveFS mds: update copyset condidate.](https://github.com/opencurve/curve/pull/1085)
+
+- [CurveFS client: fix bug of rpc overtime time not backoff.](https://github.com/opencurve/curve/pull/1139)
+
+- [CurveFS client: fix bug when upload failed.](https://github.com/opencurve/curve/pull/1130)
+
+- [CurveFS client: fix the problem that chunkCacheManager has been releaed.](https://github.com/opencurve/curve/pull/1179)
+
+- [CurveFS: fix partition num in topology metric.](https://github.com/opencurve/curve/pull/1187)
+
+- [CurveFS client: fix core dump of DataCache::Flush & fix rpc timeout not backoff.](https://github.com/opencurve/curve/pull/1193)