Update contributing guide

After Kotlin-IDE-plugin is moved to intellij repository, this commit
updates contributing.md guide with corresponding references.
Other than that brief information on current repository is provided.

docs/contributing.md

@@ -1,3 +1,25 @@
+# What this repository is
+
+This repository contains sources of:
+1. Kotlin compiler: frontend and jvm, js, wasm and native backends.
+2. Plugins extending compiler functionality.
+3. Kotlin standard library and others including
+    * kotlin-test
+    * kotlinx-metadata
+    * kotlin-reflect 
+4. Build systems support: Gradle (incl. Dukat), Maven, JPS and Ant. 
+5. Kotlin scripting support.
+
+# What this repository is not
+
+There are other activities around the language residing if different repositories.
+
+* Kotlin language support in [IntelliJ Kotlin plugin](https://plugins.jetbrains.com/plugin/6954-kotlin) comprising the compiler and IDE specifics (code highlighting,
+intentions, inspections, refactorings, etc.).\
+Starting from IntelliJ version `2021.2` its code is a part of
+[JetBrains/intellij](https://github.com/JetBrains/intellij-community) repository.\
+Platforms `2021.1` and earlier get plugin built from [JetBrains/intellij-kotlin](https://github.com/JetBrains/intellij-kotlin).
+
 # Contributing
 
 We love contributions! There is [a lot to do on Kotlin](https://youtrack.jetbrains.com/issues/KT) and on the
@@ -36,11 +58,6 @@ git config --global pull.rebase true
 This will avoid your local repo having too many merge commits
 which will help keep your pull request simple and easy to apply.
 
-## Intention/QuickFix/Inspection
-
-Please read the [Intention/QuickFix/Inspection Quick Notes](intentions_inspections_quickfixes.md) if you are
-going to implement an inspection or intention or add a quick fix.
-
 ## Checklist
 
 Before submitting the pull request, make sure that you can say "YES" to each point in this short checklist: