[GR-21011] Add README.md.

PullRequest: js/1367

Author: wirthi

README.md

@@ -1,4 +1,4 @@
-[![Join the chat at https://gitter.im/graalvm/graaljs](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/graalvm/graaljs?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![https://graalvm.slack.com](https://img.shields.io/badge/slack-join%20channel-active)](https://www.graalvm.org/slack-invitation/)
 
 A high performance implementation of the JavaScript programming language.
 Built on the GraalVM by Oracle Labs.

docs/user/JavaInterop.md

@@ -30,10 +30,13 @@ The method `Java.addToClasspath()` can be used to programmatically add to the cl
 
 ### Polyglot Context
 The preferred method of launching GraalVM JavaScript with Java interop support instance is via polyglot `Context`.
-For that, a new `org.graalvm.polyglot.Context` is built with the `hostAccess` option set:
+For that, a new `org.graalvm.polyglot.Context` is built with the `hostAccess` option allowing access and a `hostClassLookup` predicate defining the Java classes you allow access to:
 
 ```java
-Context context = Context.newBuilder("js").allowHostAccess(true).build();
+Context context = Context.newBuilder("js").
+    allowHostAccess(HostAccess.ALL).
+    allowHostClassLookup(className -> true).  //allows access to all Java classes
+    build();
 context.eval("js", jsSourceCode);
 ```
 

docs/user/README.md

@@ -0,0 +1,13 @@
+GraalVM includes an ECMAScript compliant JavaScript engine. It is designed to be
+fully standard compliant, execute applications with high performance, and
+provide all the benefits from the GraalVM stack, including language
+interoperability and common tooling. With that engine, GraalVM can execute plain
+JavaScript code or run unmodified Node.js applications. Applications can import
+npm modules, including native ones.
+
+This user documentation provides information on available GraalVM JavaScript
+engine configuration, Node.JS runtime, the `javax.script.ScriptEngine`
+implementation, multithreading support details, GraalVM JavaScript execution on
+a stock JVM like OpenJDK, possible embedding scenarios and other. To
+migrate the code previously targeted to the Nashorn or Rhino engines, migration
+guides are available.