Update README with compatibility notes for PHP 5.3.6 and earlier.
Update README with compatibility notes for PHP 5.3.6 and earlier.

unikent/splunk-sdk-php,shakeelmohamed/splunk-sdk-php,splunk/splunk-sdk-php,shakeelmohamed/splunk-sdk-php,splunk/splunk-sdk-php,unikent/splunk-sdk-php,shakeelmohamed/splunk-sdk-php,unikent/splunk-sdk-php,splunk/splunk-sdk-php