Add new sections explaining how to tune MySQL and Postgres for high traffice env
Add new sections explaining how to tune MySQL and Postgres for high traffice env

[finishes #155949688]

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release