Add minimal README

Author: realityforge

README.md

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+# React4j-Widget
+
+[![Build Status](https://secure.travis-ci.org/react4j/react4j-windowportal.png?branch=master)](http://travis-ci.org/react4j/react4j-windowportal)
+[<img src="https://img.shields.io/maven-central/v/org.realityforge.react4j.windowportal/react4j-windowportal.svg?label=latest%20release"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22org.realityforge.react4j.windowportal%22)
+[![codecov](https://codecov.io/gh/react4j/react4j-windowportal/branch/master/graph/badge.svg)](https://codecov.io/gh/react4j/react4j-windowportal)
+
+This library provides a portal that renders into a Window.
+
+## Quick Start
+
+The simplest way to use component;
+
+* add the following dependencies into the build system. i.e.
+
+```xml
+<dependency>
+   <groupId>org.realityforge.react4j.windowportal</groupId>
+   <artifactId>react4j-windowportal</artifactId>
+   <version>0.01</version>
+</dependency>
+```
+
+* add the snippet `<inherits name="react4j.windowportal.WindowPortal"/>` into the .gwt.xml file.
+
+* Use the WindowPortal as a react component.
+
+# More Information
+
+For more information about component, please see the [Website](https://react4j.github.io/react4j-windowportal). For the
+source code and project support please visit the [GitHub project](https://github.com/react4j/react4j-windowportal).
+
+# Contributing
+
+The component was released as open source so others could benefit from the project. We are thankful for any
+contributions from the community. A [Code of Conduct](CODE_OF_CONDUCT.md) has been put in place and
+a [Contributing](CONTRIBUTING.md) document is under development.
+
+# License
+
+The component is licensed under [Apache License, Version 2.0](LICENSE).