Add Gradle Init Script to Documentation?

The configuration phase to appropriately run dependency-guard is very expensive. An alternate option to having it "always installed" is to run it via a Gradle init script.

In this example, it will run the `dependencyGuardBaseline` tasks for all projects that have the plugin applied. The code iterates through all projects and only applies it to those who are `com.android.library` or `com.android.application`. This can be modified as desired.

`dependency-guard.gradle`
```groovy
/**
* Gradle init-script which applies the plugin to existing Gradle projects.
*
* Run it with the following:
* ./gradlew --init-script dependency-guard.gradle dependencyGuardBaseline
*/
settingsEvaluated {
rootProject {
buildscript {
repositories {
mavenCentral()
gradlePluginPortal()
google()
mavenLocal()
}

dependencies {
classpath("com.dropbox.dependency-guard:dependency-guard:0.4.3")
}
}
afterEvaluate {
allprojects {
project.afterEvaluate {
// filtering for android library and app plugins in this case, but you could change this logic as needed.
if (plugins.findPlugin("com.android.library") != null || plugins.findPlugin("com.android.application") != null) {
plugins.apply("com.dropbox.dependency-guard")
dependencyGuard {
configuration("debugRuntimeClasspath") {
modules = true
}
}
}
}
}
}
}
}
```

I can then just run `./gradlew --init-script dependency-guard.gradle dependencyGuardBaseline`

Additionally I can target a project specifically with`./gradlew --init-script dependency-guard.gradle :app:dependencyGuard` if desired. (to avoid configuring all projects when configuration on demand is enabled)