Add required PHP version to requirements in README.
Add required PHP version to requirements in README.

jslyonnais/SEOstats,hslatman/SEOstats,MurgaNikolay/SEOstats,WebsterFolksLabs/SEOstats,MurgaNikolay/SEOstats,WebsterFolksLabs/SEOstats,jslyonnais/SEOstats,hslatman/SEOstats,eyecatchup/SEOstats