Small "Introduction and setup" section added

Author: elizarov

coroutines-guide.md

@@ -34,6 +34,17 @@ class GuideTest {
 
 This is a short guide on core features of `kotlinx.coroutines` with a series of examples.
 
+## Introduction and setup
+
+Kotlin, as a language, provides only minimal low-level APIs in its standard library to enable various other 
+libraries to utilize coroutines. Unlike many other languages with similar capabilities, `async` and `await`
+are not keywords in Kotlin and are not even part of its standard library.
+
+`kotlinx.coroutines` in one such rich library. It contains a number of high-level 
+coroutine-enabled primitives that this guide covers, including `async` and `await`. 
+You need to add a dependency on `kotlinx-coroutines-core` module as explained 
+[here](README.md#using-in-your-projects) to use primitives from this guide in your projects.
+
 ## Table of contents
 
 <!--- TOC -->