Updated composer install instructions

Avoids suggesting people install with `*` which is kinda dangerous, and still installs the latest version to their `composer.json`.
editing the json file is not really pro, its not recommended. `composer require` is recommended because if bundles all related commands you need to run into one and does a precise "incision". It makes ofr a lot less error prone actions and helps you figure out the right version and restraint for your setup. This is surely not about lazy and pro, but letting the tool do its job, not you do it.