Added documentation for HP Vertica and Redshift

Axel, I added documentation for Vertica and Redshift, per your request last week. This documentation corresponds to the database support added in the following pull requests:

https://github.com/flyway/flyway/pull/740
https://github.com/flyway/flyway/pull/743

I tested the site in jekyll on my computer, and it looks okay to me. However, please let me know if I missed anything :)