Merge branch 'doc-for-config' into 'master'

Add some clarification for some files under config/*

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

See merge request !4851

config/README.md

+# Configuration files Documentation
+
+Note that most configuration files (`config/*.*`) committed into
+[gitlab-ce](https://gitlab.com/gitlab-org/gitlab-ce) **will 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 can find most of GitLab configuration settings here.
+
+## mail_room.yml
+
+This file is actually an YML wrapped inside an ERB file to enable templated
+values to be specified from `gitlab.yml`. mail_room loads this file first as
+an ERB file and then loads the resulting YML as its configuration.
+
+## resque.yml
+
+This file is called `resque.yml` for historical reasons. We are **NOT**
+using Resque at the moment. It is used to specify Redis configuration
+values instead.