[Fix] README for mmseg 1.x (#2009)

.github/CONTRIBUTING.md

README.md

README_zh-CN.md

docs/en/advanced_guides/add_dataset.md

docs/en/index.rst

docs/en/notes/CONTRIBUTING.md

docs/en/overview.md

docs/en/user_guides/1_config.md

docs/en/user_guides/visualization.md

docs/zh_cn/index.rst

docs/zh_cn/migration.md

docs/zh_cn/overview.md