Add a gradle configuration guide

[LeoColman]

In the Java/Kotlin world, some people use Gradle instead of maven.

It would be pretty nice to have a small guide on how to use kotlin-oauth2-server with gradle.

[velastroll]

build.gradle file:

repositories {
    maven { url 'https://dl.bintray.com/adhesivee/oauth2-server' }
}

dependencies {
    compile "nl.myndocs:oauth2-server-core:$myndocs_oauth_version"
    compile "nl.myndocs:oauth2-server-client-inmemory:$myndocs_oauth_version"
    compile "nl.myndocs:oauth2-server-identity-inmemory:$myndocs_oauth_version"
    compile "nl.myndocs:oauth2-server-token-store-inmemory:$myndocs_oauth_version"
    compile "nl.myndocs:oauth2-server-ktor:$myndocs_oauth_version"
}

version: 0.5.0

[LeoColman]

Sure, this should be added to the main documentation!