Fix createIndex change

After your changes to `List<String>`, it no longer worked for me. My intention is to use it with changelog file like so:
`<createIndex includedColumns="column1, column2">`. This is more like how liquibase-core is designed.
- Cleanup (removed superfluous comments).
- Changed param name from "includedColumnNames" to "includedColumns".
- Changed includedColumns from `List<String>` to just `String`. Trying to match the liquibase core way of doing things. The `List<String>` was just giving me a NPE when trying to use.
- Added docs to README