Add a detailed README

[boformer]

• How to use the template
  • Fork the project vs. download zip
  • All neccesary changes to folders and config values
  • license
  • readme
  • credentials
  • settings.gradle + subproject folders
  • package names
  • maven group + artifactIds
• Versioning
• License header generation
• How to build the plugin
• How to add an implementation folder for Bukkit
  • dependencies
  • settings.gradle
• advanced setup for cross-platform plugins (maybe outsource this to an advanced template?)
  • -api folder (optional)
  • -core folder
  • -sponge, -bukkit folder
• How to deploy the API to a maven repo
  • setting the mavenUrl in {user}/.gradle/gradle.properties
  • other ways