Add documentation and example file for environent variables.

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+# Environment Variables
+
+## Introduction
+
+Commonly people configure GitLab via the gitlab.rb configuration file in the Omnibus package.
+
+But if you prefer to use environment variables we allow that too.
+
+## Supported environment variables
+
+Variable | Type | Explanation
+--- | --- | ---
+GITLAB_ROOT_PASSWORD | string | sets the password for the `root` user on installation
+GITLAB_HOST | url | hostname of the GitLab server includes http or https
+RAILS_ENV | production/development/staging/test | Rails environment
+DATABASE_URL | url | For example: postgresql://localhost/blog_development?pool=5
+
+## Complete database variables
+
+As explained in the [Heroku documentation](https://devcenter.heroku.com/articles/rails-database-connection-behavior) the DATABASE_URL doesn't let you set:
+
+- adapter
+- database
+- username
+- password
+- host
+- port
+
+To do set these please `cp config/database.yml.rb config/database.yml` and use the following variables:
+
+Variable | Default
+--- | ---
+GITLAB_DATABASE_ADAPTER | postgresql
+GITLAB_DATABASE_ENCODING | unicode
+GITLAB_DATABASE_DATABASE | gitlab_#{ENV['RAILS_ENV']
+GITLAB_DATABASE_POOL | 10
+GITLAB_DATABASE_USERNAME | root
+GITLAB_DATABASE_PASSWORD |
+GITLAB_DATABASE_HOST | localhost
+GITLAB_DATABASE_PORT | 5432
+
+## Other variables
+
+We welcome merge requests to make more settings configurable via variables.
+Please stick to the naming scheme "GITLAB_#{name 1_settings.rb in upper case}".