docs: Add PHP compatibility table to README

[mtrezza]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

It's unclear for developers which PHP version are supported.

Approach

Add PHP compatibility table to README.

[parse-github-assistant]

Thanks for opening this pull request!

• ❌ Please link an issue that describes the reason for this pull request, otherwise your pull request will be closed. Make sure to write it as Closes: #123 in the PR description, so I can recognize it.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (77b06f4) 98.60% compared to head (3c1756e) 98.60%.

❗ Current head 3c1756e differs from pull request most recent head 0ac1e58. Consider uploading reports for the commit 0ac1e58 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #514   +/-   ##
=========================================
  Coverage     98.60%   98.60%           
  Complexity     1062     1062           
=========================================
  Files            38       38           
  Lines          3229     3229           
=========================================
  Hits           3184     3184           
  Misses           45       45           

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[parseplatformorg]

🎉 This change has been released in version 2.2.0