[QA] add phpstan

[f3l1x]

Configuration in the README (https://github.com/phpstan/phpstan).

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cc @ondrejmirtes

[ondrejmirtes]

@f3l1x I can prepare a PR if you'd like me to.

[f3l1x]

@ondrejmirtes Thanks a lot, but it not nescessary. I would like to go throw the readme and handle possible breakpoints by my self. I can provide some feedback after that.

Thank you for your jof on @phpstan. It looks like a great missing tool for PHP world. 👍

[ondrejmirtes]

@f3l1x OK, just show me the integration before merging so I can give my 2 cents on it :)