[README] Add deprecation notice and link to PHPStan rule set

[TomasVotruba]

This is not a normal deprecation, but rather a replacement 1:1 with better tool.

PHP_CodeSniffer is great for handling spaces and char positions. Yet these rules are about code architecture and structure. In 2020, there is tool that suits this perfectly - PHPStan.

Saying that, object calisthenics were implemented as PHPStan rules in a symplify/coding-standard package. It should be used instead.

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cc @guilhermeblanco Hi Guilherme, I know you're not active much here, but still I think you should know. Let me know if this move is ok for you.
Not much really changes, rules just work much better now as PHPStan doesn't deal with spacing bugs.

[guilhermeblanco]

@TomasVotruba I agree with the statement. PHPStan is simple to adopt and better suited for these type of checks.
I just have mixed feelings on switching organizations instead of opening a new repository in this one. However, since you've been the main contributor of this repository for a while, I trust your judgement.

Please let me know if there's anything I can do to assist you in this change.

[TomasVotruba]

Thanks for reaching out so quickly!

I just have mixed feelings on switching organizations instead of opening a new repository in this one.

I'll try to explain why I belieive this is the best.

1. One Location

I mentioned there are few many new repositories with PHPStan rules. Actually, there are so many of them, that it's very hard to "Google" them on Github. Each person has his or her small set of PHPStan rules, but they can't be found. I saw some rule in June, but I can't find it now, so I have to reimplement it.

2. Maintenance

While it's quite easy and optimistic to maintain a repositories for 1st or 2nd year of existance, later it becomes obsolete. The more split the packages and maintainer attention is, the faster it rottens. Saying that, I've chosen the main&only monorepo I maintain - https://github.com/symplify/symplify

I know that even if I don't have time for any other projets and repositories, this I'll mostly likely maintain the longest.

I was thinking about making symplify/object-calisthenics-phpstan-rules, but it would only cause both mentioned troubles. From what I can observe for last 7 years in open-source maintenance world, this should make this package the most health and stability.

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Please let me know if there's anything I can do to assist you in this change.

I think I have all the access I need - on packagist I marked the redirect:
https://packagist.org/packages/object-calisthenics/phpcs-calisthenics-rules

There is a little post about this change: https://tomasvotruba.com/blog/2020/09/07/introducing-object-calisthenics-rules-for-phpstan/
It would help if you could share it, to make the change more official.

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Thank you 👍