Versionable content should not be altered with custom information

[EliuX]

In the 4th step of the README.md file, the user is told to change the information of config/administrators.json with his/hers. This alternative turns the project more error prone to commit sensitive information.
This can be solved by putting a sample file that can be copied into config/administrators.json, by putting a command in one of the scripts phases in the composer.json file, maybe pre-autoload-dump, which runs before the autoloader is dumped, either during install/update, or via the dump-autoload command.