docs: added changelog v2.6.

Signed-off-by: Wine93 <wine93.info@gmail.com>

CHANGELOG-2.6.md

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+# CHANGELOG of v2.6
+
+Previous change logs can be found at [CHANGELOG-2.5](https://github.com/opencurve/curve/blob/master/CHANGELOG-2.5.md)
+
+## Features
+
+* [curvefs/client: add qos for curve-fuse, we can limits the bandwidth](https://github.com/opencurve/curve/pull/2424), thanks to [@UniverseParticle](https://github.com/UniverseParticle).
+* [curvefs/client:add memcache to warmup](https://github.com/opencurve/curve/pull/2270), thanks to [@Cyber-SiKu](https://github.com/Cyber-SiKu).
+
+## Improve
+
+* [curvefs/client: improve metadata performance](https://github.com/opencurve/curve/pull/2387), thanks to [@Wine93](https://github.com/Wine93).
+* [curvefs/client: change the data organization format](https://github.com/opencurve/curve/pull/2420), thanks to [@201341](https://github.com/201341).
+* [curvefs/client: optimize DiskCacheManager::UmountDiskCache()](https://github.com/opencurve/curve/pull/2252), thanks to [@Ziy1-Tan](https://github.com/Ziy1-Tan).
+* [curvefs/client: add memcached startup check](https://github.com/opencurve/curve/pull/2296), thanks to [@Ziy1-Tan](https://github.com/Ziy1-Tan).
+
+## Bugfix
+
+* [curvefs/client: enableSumInDir in multi-mount situation](https://github.com/opencurve/curve/pull/2491), thanks to [@bit-dance](https://github.com/bit-dance).
+* [curvefs/client: fuse client exit when cache size less than 8MB](https://github.com/opencurve/curve/pull/2492), thanks to [@Ziy1-Tan](https://github.com/Ziy1-Tan).
+* [curvefs/client: remove unused setting fuseMaxSize](https://github.com/opencurve/curve/pull/2497), thanks to [@Ziy1-Tan](https://github.com/Ziy1-Tan).
+* [curvefs/client: enable async read for FileCacheManager::ReadKVRequest](https://github.com/opencurve/curve/pull/2421), thanks to [@Ziy1-Tan](https://github.com/Ziy1-Tan).
+
+
+# Performance
+
+## Hardware
+3 nodes (3*mds, 9*metaserver), each with:
+ - Intel(R) Xeon(R) CPU E5-2680 v4 @ 2.40GHz
+ - 256G RAM
+ - disk cache: INTEL SSDSC2BB80 800G (IOPS is about 30000+, bandwidth is about 300 MiB)
+
+## Configure
+
+```yaml
+fs.cto: true
+fs.lookupCache.negativeTimeoutSec: 1
+fs.lookupCache.minUses: 3
+fuseClient.supportKVcache: true
+client.loglevel: 0
+```
+
+## fio
+
+```bash
+[global]
+rw=randread
+direct=1
+size=50G
+iodepth=128
+ioengine=libaio
+bsrange=4k-4k
+ramp_time=10
+runtime=300
+group_reporting
+
+[disk01]
+filename=/path/to/mountpoint/1.txt
+```
+
+| fio | IOPS/bandwidth | avg-latency(ms) | clat 99.00th (ms) | clat 99.99th (ms) |
+| :----: | :----: | :----: | :----: | :----: |
+| numjobs=1 / size=50GB / 4k randwrite | 4243      | 0.23 | 0.176 | 2   |
+| numjobs=1 / size=50GB / 4k randwrite | 908       | 1.0  | 3.5   | 104 |
+| numjobs=1 / size=50GB / 512k write   | 412 MiB/s | 2.4  | 19    | 566 |
+| numjobs=1 / size=50GB / 512k read    | 333 MiB/s | 2.9  | 20    | 115 |
+
+## mdtest
+
+```bash
+for i in 1 4 8; do mpirun --allow-run-as-root -np $i mdtest -z 2 -b 3 -I 10000 -d /path/to/mountpoint; done
+```
+
+| Case | Dir creation | Dir stat | Dir removal | File creation | File stat | File read | File removal | Tree creation | Tree removal |
+| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
+| client*1 | 341 | 395991 | 291 | 334 | 383844 | 3694 | 309 | 322 | 851 |
+| client*4 | 385 | 123266 | 288 | 361 | 1515592 | 15056 | 310 | 363 | 16 |
+| client*8 | 415 | 22138 | 314 | 400 | 2811416 | 20976 | 347 | 355 | 8 |