Documentation improvements 📃

[mvicsokolova]

Users tend to have a lot of questions regarding plugin application: how to set up the legacy gradle plugin application via buildscript classpath configurations or whether they should apply the compiler atomicfu plugin directly.

So, it'd be great to make the application as easy as possible and document it properly.
The roadmap can be as follows:

1. Publish Gradle plugin to https://plugins.gradle.org #56
2. Improve atomicfu gradle extension  #341

After these improvements the plugin application should be clean and easy, just by the plugin id.

• Make it clear how to apply the plugin and how to enable compiler plugin transformations.
• Add examples of gradle build scripts to the readme, that can be copy-pasted to the project and work.
• Write down all the compiler plugin constraints in the readme (those from CONSTRAINTS_ERROR_MESSAGE)
• Add code samples to the README (for atomic properties, arrays, delegation ...)
• Add lacking documentation to all the public API.
• It's be nice to generate dokka documentation and publish it on the site.