Provide a link to additional starters in Help menu

The list of starters/dependencies available for project generation on start.spring.io is curated based on a number of factors. There are additional community starters that we could help discover by pointing to [this](https://github.com/spring-projects/spring-boot/blob/b0626bca320c21928e9790fcd2c59337344f9ac5/spring-boot-project/spring-boot-starters/README.adoc).