Describe how to add kotlin support for existing java project

[alexander-shustanov]

1. Update the project version to 7.2+.
2. Open build.gradle
3. In buildScript section add ext.kotlinVersion = '1.3.41' line.
4. In buildScript -> dependencies subsection add following lines:

   classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
   classpath "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlinVersion"

5. Before cuba section add apply(plugin: 'org.jetbrains.kotlin.jvm') line.
6. Find configure([globalModule, coreModule, webModule]) section. It may contain more modules.
7. After apply(plugin: 'cuba') line add following: apply(plugin: 'org.jetbrains.kotlin.jvm').
8. In dependencies subsections add:

   compile("org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion")
   compile("org.jetbrains.kotlin:kotlin-reflect:$kotlinVersion")