Update composer install instructions to use require

It's recommended that `composer require` be used for installing
dependencies instead of manually altering the `composer.json` file.

Installation steps are now much simpler, only the one require step,
instead of requiring people to run `composer update` and `composer
install` manually.

This came up in [a discussion](https://github.com/KnpLabs/snappy/pull/118#discussion_r20793023)
in another project and I figured it would be good to adjust here as
well.
Merge pull request #12 from site5/documentation/ComposerRequire

Update composer install instructions to use require