Add required PHP version go readme

[westonruter]

Fixes #159.

See https://make.wordpress.org/plugins/2017/08/29/minimum-php-version-requirement/

[valendesigns]

@westonruter Shouldn't this be updated in the readme generator in dev-lib?

[westonruter]

@valendesigns updated in what way?

[westonruter]

The readme generator is copying “Requires PHP” into the markdown version.

[valendesigns]

Add Requires PHP to this array https://github.com/xwp/wp-dev-lib/blob/master/class-wordpress-readme-parser.php#L41 and inject it into the generated readme.md

[westonruter]

It doesn't seem to be required because it works without it.

[valendesigns]

I see, so the readme generator is automatically adding Requires PHP even though it's not mentioned in the file anywhere. Awesome!

[westonruter]

I think the predefined list in the script just ensures that the same order is used when generated.