Add contributing guidelines
Add contributing guidelines

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