Bump version to v0.23.0 (open-mmlab#1439)

* Bump version to v0.23.0

* new contributor

* rephrase

* tyoi

* typo

* get_started

* typo

* rephrase

* rephrase

* rephrase

Bump v0.22.1 (open-mmlab#1361)

Bump v0.22.0 (open-mmlab#1347)

* change version to v0.22.0

* change version to v0.22.0

* add mmcls version in get_started.md

* add mmcls installation and move PR1299 into enhancement

* add mmcls installation and move PR1299 into enhancement

* remove MMCLS and make mmcv <=1.5.0 version in get_started.md

* fix typo

Bump v0.21.1 (open-mmlab#1273)

* Bump v0.21.1

* add improvements in changelog

* add improvements in changelog

* fix cn readme

* change changelog

Bump v0.21.0 (open-mmlab#1258)

* change version to v0.21.0

* change version to v0.21.0

* change version to v0.21.0

* change version to v0.21.0

Bump v0.20.2 open-mmlab#1141

Bump to v0.20.1 (open-mmlab#1138)

* bump version 0.20.1

* bump version 0.20.1

Bump v0.20.0 (open-mmlab#1124)

* change version to v0.20.0

* change version to v0.20.0

* change version to v0.20.0

Bump v0.19.0 (open-mmlab#1009)

* change version to v0.19.0

* update changelog

Bump to v0.18.0 (open-mmlab#940)

* bump to v0.18.0

* replace \ with /