README.md: add some basic information

including a not too primitive example

Change-Id: I4da4cecf857a71eac66937a7a1cc38e9fdfe485d

Author: Thomas Singer

README.md

@@ -1,5 +1,81 @@
 [![SWT Matrix Build](https://github.com/eclipse-platform/eclipse.platform.swt/actions/workflows/maven.yml/badge.svg)](https://github.com/eclipse-platform/eclipse.platform.swt/actions/workflows/maven.yml) ![SWT Matrix Tests](https://gist.githubusercontent.com/eclipse-releng-bot/78d110a601baa4ef777ccb472f584038/raw/71510599eb84e852f3e135aa7a3ddf33854ca716/badge.svg)
 
+# About
+
+SWT is a cross-platform GUI library for JVM based desktop applications.
+The best known SWT-based application is [Eclipse](https://www.eclipse.org).
+
+## Getting Started
+
+SWT comes with platform-specific jar files.
+Download them from https://download.eclipse.org/eclipse/downloads/index.html and add the jar file to your classpath.
+
+### Example
+![Example](example.png)
+```java
+import org.eclipse.swt.SWT;
+import org.eclipse.swt.layout.GridData;
+import org.eclipse.swt.layout.GridLayout;
+import org.eclipse.swt.widgets.Button;
+import org.eclipse.swt.widgets.Display;
+import org.eclipse.swt.widgets.Label;
+import org.eclipse.swt.widgets.Shell;
+import org.eclipse.swt.widgets.Text;
+
+public class HelloWorld {
+
+	public static void main(String[] args) {
+		final Display display = new Display();
+
+		final Shell shell = new Shell(display);
+		shell.setText("Hello World");
+		shell.setLayout(new GridLayout(2, false));
+
+		final Label label = new Label(shell, SWT.LEFT);
+		label.setText("Your &Name:");
+		label.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, false, false));
+
+		final Text text = new Text(shell, SWT.BORDER | SWT.SINGLE);
+		final GridData data = new GridData(SWT.FILL, SWT.CENTER, true, false);
+		data.minimumWidth = 120;
+		text.setLayoutData(data);
+
+		final Button button = new Button(shell, SWT.PUSH);
+		button.setText("Say Hello");
+		shell.setDefaultButton(button);
+		button.setLayoutData(new GridData(SWT.END, SWT.CENTER, false, false, 2, 1));
+
+		final Label output = new Label(shell, SWT.CENTER);
+		output.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, true, false, 2, 1));
+
+		button.addListener(SWT.Selection, event -> {
+			String name = text.getText().trim();
+			if (name.length() == 0) {
+				name = "world";
+			}
+			output.setText("Hello " + name + "!");
+		});
+
+		shell.setSize(shell.computeSize(SWT.DEFAULT, SWT.DEFAULT));
+		shell.open();
+
+		while (!shell.isDisposed()) {
+			if (!display.readAndDispatch()) {
+				display.sleep();
+			}
+		}
+
+		display.dispose();
+	}
+}
+```
+First, a `Display` is created which is something like the central place of all GUI-related code.
+Then a `Shell` is created which in our example is a top-level window.
+Then all child controls and listeners are created, including their layout information.
+Finally, we set the window's size determines by its child controls and open the window.
+The `while`-loop processes all GUI related events until the shell is disposed which happens when closing.
+Before exiting, any claimed GUI resources needs to be freed.
+
 Contributing to SWT
 ===================