update readme installation rules

[przemekhernik]

Description

Since the project has been moved to PHPCSStandards organization, we need start updating docs. I feel that the most important are installation rules, so I've updated composer commands to use the new version. It's not too much, but at least something to start :)

Suggested changelog entry

No need for changelog entry.

Related issues/external references

none

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.

[jrfnl]

@przemekhernik I appreciate that you are trying to contribute to the project, but you are basing this PR on outdated information.

The change you are proposing was originally already made in the repo and has been reverted.

In contrast to earlier information, arrangements have been made to allow the package to continue under its original name on Packagist.

The information in the "The future of" post in the squizlabs repo as well as in this repo was updated to reflect this last week and this information is also very explicitly included in the release notes of the 3.8.0 release.

I'm going to close this PR now.