Requires related plugin header check in readme

[ernilambar]

Related: https://github.com/WordPress/plugin-check/issues/198

For following two headers, we accept fields either in readme file or main plugin file.

• Requires at least
• Requires PHP

We could raise ERROR or WARNING if those fields are missing from both files.

It seems preference is given to plugin file header. May be we could create separate Check instead of keeping inside plugin_readme check.