Actualize ReadMe files

- Actualize build requirements and steps
- Extract Gradle plugin readme to the corresponding folder
- Rewrite Libraries and Maven Tools readme

#KT-20995 Fixed

libraries/tools/kotlin-gradle-plugin/ReadMe.md

@@ -0,0 +1,26 @@
+## Gradle Plugin
+
+The Gradle plugin sources can be found in this ([kotlin-gradle-plugin](./)) module.
+
+To install the Gradle plugin into the local Maven repository, run this command from the root of Kotlin project:
+
+    ./gradlew :kotlin-gradle-plugin:install
+    
+The subplugin modules are `:kotlin-allopen`, `:kotlin-noarg`, `:kotlin-sam-with-receiver`. To install them, run:
+
+    ./gradlew :kotlin-allopen:install :kotlin-noarg:install :kotlin-sam-with-receiver:install
+
+To find more details about the plugins provided by this artifact and their tasks refer to [Module.md](Module.md).
+
+### Gradle integration tests
+
+Gradle integration tests can be found at the [kotlin-gradle-plugin-integration-tests](../kotlin-gradle-plugin-integration-tests) module.
+
+Run the integration tests from the root project:
+
+    ./gradlew :kotlin-gradle-plugin-integration-tests:test
+    
+The tests that use the Gradle plugins DSL ([`PluginsDslIT`](../kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/PluginsDslIT.kt)) also require the Gradle plugin marker artifacts to be installed:
+
+    ./gradlew -Pdeploy_version=1.2-test :kotlin-gradle-plugin:plugin-marker:install :kotlin-noarg:plugin-marker:install :kotlin-allopen:plugin-marker:install
+    ./gradlew -Pdeploy_version=1.2-test :kotlin-gradle-plugin-integration-tests:test