Update README.md.

Author: gk-brown

README.md

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-# Sierra
-Declarative UI for Java
+[![Releases](https://img.shields.io/github/release/HTTP-RPC/Sierra.svg)](https://github.com/HTTP-RPC/Sierra/releases)
+[![Maven Central](https://img.shields.io/maven-central/v/org.httprpc/sierra.svg)](https://repo1.maven.org/maven2/org/httprpc/sierra/)
+
+# Introduction
+Sierra is an open-source framework for simplifying development of Java Swing applications. The project's name comes from the nautical _S_ or _Sierra_ flag:
+
+![](sierra.png)
+
+This guide introduces the Sierra framework and provides an overview of its key features.
+
+# Contents
+* [Getting Sierra](#getting-sierra)
+* [Sierra Classes](#sierra-classes)
+* [Additional Information](#additional-information)
+
+# Getting Sierra
+Sierra is distributed via Maven Central:
+
+* [org.httprpc:sierra](https://repo1.maven.org/maven2/org/httprpc/sierra/) - TODO (Java 11 or later required)
+
+# Sierra Classes
+Classes provided by the Sierra framework include:
+
+TODO
+
+# Additional Information
+TODO