add link to circleci config in README (#8089)
add link to circleci config in README (#8089)

Signed-off-by: Ricky Pai <4ca84b65e96ac2ab68658335deeeb8ad762e2cdd@gmail.com>
keegancsmith/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus