Improve plugin documentation

[sivaprasadreddy]

Currently the README is acting as the plugin documentation with minimal details.

Need to improve the plugin documentation by adding the details of:

• What are the supported databases, their default docker images
• What database migration tools are supported
• Supported Flyway configuration parameters
• Supported Liquibase configuration parameters
• Supported jOOQ configuration parameters

[romchellis]

I think we can close it once #19 completed