README.md update and format tweaks

The section for mysql::client didn't specify that package_name and package_enable were available parameters.

Also took the liberty of making the override_options sections identical between ###Overrides and ###Parameters->mysql::server->override_options, as well as marking various other items as `code`, for a prettier display.
Merge pull request #317 from StephenBrown2/patch-1

README.md mysql::client parameters update and format tweaks