[Docs] Request to supply some comment or documentation

Branch

master branch https://mmdetection.readthedocs.io/en/latest/

📚 The doc issue

I found no comment or documentations about config files, which is inconvient. For example, what's the main difference between files in configs/common?

Suggest a potential alternative/fix

A short summary in this directory as readme.md is fine. If the information is maintained at other places, it will be appreciated to indicate where it is.