[Serialization] Reorganize module structure

plugins/kotlinx-serialization/README.md

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+# Kotlin serialization IDEA plugin
+
+Kotlin serialization plugin consists of three parts: a compiler plugin, an IntelliJ plugin and a runtime library.
+This is the folder with common source for all plugins, IDEA plugin is built from here. Gradle and Maven plugins can be found in `libraries` folder.
+
+## Building and usage
+
+### Prerequisites:
+Before all, follow the instructions from root README.md to download dependencies and build Kotlin compiler. (`./gradlew dist`)
+
+**Plugin works only with IntelliJIDEA 2017.2 and higher.**
+
+Make sure you have latest dev version of Kotlin plugin installed.
+
+### With gradle:
+
+Run `./gradlew :kotlinx-serialization-compiler-plugin:dist`.
+In IDEA, open `Settings - Plugins - Install plugin from disk...` and choose `$kotlin_root/dist/artifacts/Serialization/lib/kotlinx-serialization-compiler-plugin.jar`
+
+### From within IDE (for development):
+
+Run `./gradlew runIde` You'll get a fresh copy of IDEA with Kotlin and Kotlin-serialization plugins built from sources.
+
+## Building gradle plugin
+
+Run `./gradlew :kotlinx-gradle-serialization-plugin:install`
+
+## Building maven plugin
+
+Make all prerequisites from libraries' README.md for Maven projects. Go to `$kotlin_root/libraries/tools/kotlin-maven-serialization`. Run `mvn install`
+