Display custom readme for a project

Display a custom file as project's default readme.

It would be useful to have a way to define which file is displayed as the project's default readme. Such a file may be located in a subdirectory, not necessarily in the root.

For example after cloning a repository one may like to display the changes between the cloned and the current repository instead of showing the original readme.

The setting may be global for all repositories, but would be nice to be able to override it for every repository.

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/18933

~"feature proposal"