Driver install instructions

[anywarewebmarketing]

I spent a lot of time trying to figure out how to get the jdbc driver working with MySQL before I eventually got some advice that you have to install it separately. This was not clear from the docs.
How about adding some driver installation instructions to the docs?
https://www.elastic.co/guide/en/logstash/6.6/plugins-inputs-jdbc.html#_drivers

For example, change the "Drivers" section to this:
This plugin does not come packaged with JDBC driver libraries. The desired jdbc driver must be downloaded and installed separately and the jdbc driver library must be explicitly passed in to the plugin using the jdbc_driver_library configuration option.

Example driver libraries include:
MySQL: https://dev.mysql.com/downloads/connector/j/