Add some clarification for some files under config/*

I didn't add a lot of them because I am not very familiar about how they
work and I am not sure if we really need to explain files like
config/database.yml because if you know Rails you must know how
that works.

At any rate, I think this is a beginning and we could keep documenting
more files in the future.

Closes #18373

config/README.md

+# Configuration files Documentation
+
+Note that most configuration files (`config/*.*`) committed into
+[gitlab-ce](https://gitlab.com/gitlab-org/gitlab-ce) **would not be used** for
+[omnibus-gitlab](https://gitlab.com/gitlab-org/omnibus-gitlab). Configuration
+files committed into gitlab-ce are only used for development.
+
+## gitlab.yml
+
+You could find most of GitLab configuration here.
+
+## mail_room.yml
+
+It's intended to be an ERB file because `mail_room` would use ERB to evaluate
+it before parsing it as a YAML file. It would try to read values from
+`gitlab.yml` so you should configure it there.
+
+## resque.yml
+
+It's called `resque.yml` for historical reason, and we're not using rescue
+at the moment. It's served as a **Redis configuration file** instead.