make Grafana 7.x setup notes more visible in README.md
make Grafana 7.x setup notes more visible in README.md

Signed-off-by: Eugene Klimov <d71044d213981454e5e9e3fe75ed52edff8bbc7b@altinity.com>

Vertamedia/clickhouse-grafana,Vertamedia/clickhouse-grafana,Vertamedia/clickhouse-grafana,Vertamedia/clickhouse-grafana,Vertamedia/clickhouse-grafana