README: add link to the Requirements wiki page

[rodrigoprimo]

Description

As discussed in #1393, this PR adds a link to the Requirements wiki page in the README to make information about required and recommended PHP extensions more discoverable.

The existing paragraph was also split into two: one for the core PHP version and extension requirements, and another for individual sniff requirements.

Suggested changelog entry

N/A

Related issues/external references

Fixes #1393

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.
• I have opened a sister-PR in the documentation repository to update the Wiki.