update wording in readme, conveying no longer an alpha project
update wording in readme, conveying no longer an alpha project

raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,replay/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,raintank/metrictank