Updating the README
Updating the README

Version bumped to 1.3.0.  Minor update of the "Developing and testing" section.
googleapis/java-bigtable-hbase,kevinsi4508/cloud-bigtable-client,kevinsi4508/cloud-bigtable-client,sduskis/cloud-bigtable-client,googleapis/java-bigtable-hbase,sduskis/cloud-bigtable-client,googleapis/java-bigtable-hbase