Add PHPStan Template

[marcelthole]

Hi,

are you open for PHPStan specific Annotations?
The following PR will fix following use case:

$spec = cebe\openapi\Reader::readFromYamlFile($inputFile, cebe\openapi\spec\OpenApi::class);
// this is required because the method returns only a SpecObjectInterface as annotation
assert($spec instanceof cebe\openapi\spec\OpenApi);

With the @phpstan-template annotation, PHPStan knowns, that the second parameter defines the return type and the paramater must be a class string and an instanceof SpecObjectInterface.

This should help other people who uses PHPStan and your library.

If you are fine with this, i will try to add more PHPStan specific type hints. But first i wanted to know if you would support that annotations.

[cebe]

Hi @marcelthole, thanks for the suggestion. I am fine with adding these.

[cebe]

Thank you!