Docs: Add example on how to add dependencies for Java

It would be good is the Java section at least shows one way to build the jar using external dependencies.

Since we all in OpenWhisk are fans of gradle 🙄 maybe we can do one using gradle.

Here is a good example from @rabbah
https://github.com/rabbah/wsk-qr/blob/master/build.gradle
```

apply plugin: 'java'

version = '1.0'

repositories {
mavenCentral()
}

configurations {
provided
compile.extendsFrom provided
}

dependencies {
provided 'com.google.code.gson:gson:2.6.2'
compile 'com.google.zxing:core:3.3.0'
compile 'com.google.zxing:javase:3.3.0'
}

jar {
dependsOn configurations.runtime

from {
(configurations.runtime - configurations.provided).collect {
it.isDirectory() ? it : zipTree(it)
}
}
}
```
then build jar for Action
```
gradle jar
```