add banner to README to clarify version docs discrepancy
add banner to README to clarify version docs discrepancy

zahoorkhan/google-api-php-client,Jyo1/google_api_php,eRadical/google-api-php-client,ankush1990/google-analytic,googleapis/google-api-php-client,michabbb-backup/google-api-php-client,bshaffer/google-api-php-client,michabbb-backup/google-api-php-client