Make some more tweaks to docs after migrating to Maven Central

[adamdougal]

• Some steps were unable to be tested before the last PR
• This makes changes that were discovered to be required
• Update the README to show that it can be pulled from Maven Central

Required by https://github.com/sky-uk/cqlmigrate/issues/94

[adamdougal]

Closing and reopening to try and trigger the Travis build