Mention that this is not a general purpose library

Author: ggrossetie

README.adoc

@@ -1,10 +1,14 @@
 = Opal Runtime for Asciidoctor.js
+:caution-caption: :fire:
 
 ifdef::env-github[]
 image:https://img.shields.io/travis/Mogztter/asciidoctor-opal-node-runtime/master.svg[Travis build status, link=https://travis-ci.org/Mogztter/asciidoctor-opal-node-runtime]
 image:http://img.shields.io/npm/v/asciidoctor-opal-runtime.svg[npm version, link=https://www.npmjs.org/package/asciidoctor-opal-runtime]
 endif::[]
 
+[caption=Status]
+CAUTION: This library is an _highly optimized_ version of the Opal runtime that contains the *bare minimum to run Asciidoctor*. If you are looking for a general purpose Opal runtime, please use https://www.npmjs.com/package/opal-runtime.
+
 == Usage
 
 ```javascript