Update README.md

[SimonMarquis]

Add Maven version badges near the build script snippets and suffix the snapshot version in buildScript classpath example.

We could also remove the explicit version name in the main snippet and have somthing like this if you want:

---

Setup and Configuration

Step 1: Adding The Dependency Guard Gradle Plugin and Baselining

// sample/app/build.gradle.kts
plugins {
  id("com.dropbox.dependency-guard") version "x.y.z"
}