Add WP and PHP requirements policy

[kasparsd]

Is your enhancement related to a problem? Please describe.

Considering the important use-case of this plugin, users should be able to plan for keeping their infrastructure up to date with the requirements of this plugin.

Proposed Solution

Add a section to the readme.txt that documents the update policy for WP core and PHP requirements.

See #641 for relevant discussion.

= PHP and WordPress Version Support =

This plugin supports the last two major versions of WordPress and <a href="https://make.wordpress.org/core/handbook/references/php-compatibility-and-wordpress-versions/">the minimum PHP version</a> supported by those WordPress versions.

Designs

No impact.

Describe alternatives you've considered

No response

Please confirm that you have searched existing issues in this repository.

Yes

[kasparsd]

@jeffpaul Can you please suggest the exact approach and wording here?