Add link to execute the process in a browser IDE (Codenvy)

It's nice to see all the steps that we have to do, but it requires for example
- JDK 1.8 or later
- Gradle 2.3+ or Maven 3.0+
- git to clone the project or download and extract a zip file

whereas here, by using the following button, we only have to click on it ("Workspace Developer" button)
then we can try and develop spring rest service from the web browser.

README.adoc

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+image:http://beta.codenvy.com/factory/resources/codenvy-contribute.svg["Codenvy Developer Workspace", link="http://beta.codenvy.com/f?id=9fq0busbm3tz7i8c"]
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 This guide walks you through the process of creating a "hello world" link:/understanding/REST[RESTful web service] with Spring.
 
 == What you'll build