Add reference to final methods and to CS fixers in the final classes article

[Ocramius]

Ref: https://slamdunk.github.io/blog/when-to-declare-methods-final/
Ref: https://ocramius.github.io/blog/when-to-declare-classes-final/
Ref: https://disqus.com/home/discussion/ocramius-dev/when_to_declare_classes_final/?utm_source=reply&utm_medium=email&utm_content=comment_date#comment-3293493178

[TomasVotruba]

You might also like:

• https://github.com/Symplify/CodingStandard#non-abstract-class-that-implements-interface-should-be-final
• https://github.com/Symplify/CodingStandard#implementation-of-interface-should-only-contain-its-methods