Improve Composer notes in the readme

`require-dev` is better than `require` for such tool, because it is not
needed to be installed on production.

`composer update` without specifying vendor libraries as an argument
will try to update all dependencies which may not be desired.

Related to: https://github.com/andres-montanez/MagallanesSite/pull/3
Merge pull request #122 from hkdobrev/composer-readme-improvements

Improve Composer notes in the readme