Add example for parallel eval.

examples/src/org.graalvm.polyglot.examples/src/org/graalvm/polyglot/examples/ParallelEval.java

@@ -0,0 +1,30 @@
+package org.graalvm.polyglot.examples;
+
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+import org.graalvm.polyglot.Engine;
+
+/**
+ * Example shows how scripts can be executed in parallel. Please note that contexts are single
+ * threaded by design. It is allowed to use a context on more than one thread, but not at the same
+ * time.
+ */
+public class ParallelEval {
+
+    public static void main(String[] args) throws InterruptedException, ExecutionException {
+        ExecutorService service = Executors.newFixedThreadPool(1);
+        Engine engine = Engine.create();
+        Future<Integer> future = service.submit(() -> engine.getLanguage("js").createContext().eval("21 + 21").asInt());
+
+        // do work while JavaScript executes
+
+        int result = future.get();
+        assert 42 == result;
+
+        engine.close();
+    }
+
+}