Expand the available examples about rules_spring

[plaird]

@deveshkandpal24121990 I am nearly through the process of open sourcing our internal services/bazel-demo git repo, which has a few springboot examples that I use for testing the springboot rule with the three different maven rule styles. We can figure out if these examples should live here in this repo, or in the github/bazel-demo repo.

[plaird]

Our internal examples repo is now public:

https://github.com/salesforce/bazel-demo

[plaird]

We worked this in 2.1.0 with:

• DemoApp - which is intended to be the comprehensive app that shows off many features, as opposed to HelloWorld which is as simple as can be
• KotlinApp - shows how to build a Kotlin Spring Boot app