[MMSIG]Translate the documents of SlowFast & SlowOnly README document into Chinese.

[tackhwa]

Motivation

Translate the documents of SlowFast & SlowOnly README document into Chinese

Modification

add configs/detection/slowfast/README_zh-CN.md
add configs/detection/slowonly/README_zh-CN.md

Checklist

1. Pre-commit or other linting tools should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation should be modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.