add usage info to root doc page

Author: bjornregnell

src/rootdoc.txt

@@ -1,7 +1,32 @@
-This is the documentation of the `introprog` Scala library with utilities used in beginner computer science teaching at Lund University.
+This is the documentation of the `introprog` Scala library with beginner-friendly utilities used in computer science teaching at Lund University. The code repository is hosted at [[https://github.com/lunduniversity/introprog-scalalib]].
 
 == Package contents ==
 
-* [[introprog.PixelWindow `introprog.PixelWindow`]] for simple, pixel-based drawing.
+- [[introprog.PixelWindow `introprog.PixelWindow`]] for simple, pixel-based drawing.
 
-* [[introprog.PixelWindow.Event `introprog.PixelWindow.Event`]] for event management in a PixelWindow.
+- [[introprog.PixelWindow.Event `introprog.PixelWindow.Event`]] for event management in a PixelWindow.
+
+- [[introprog.IO `introprog.IO`]] for file system interaction.
+
+- [[introprog.Dialog `introprog.Dialog`]] for user interaction with standard GUI dialogs.
+
+- [[introprog.examples `introprog.examples`]] with code examples demonstrating how to use this library.
+
+== How to use this library with `sbt` ==
+
+If you have [[https://www.scala-sbt.org/ `sbt`]] installed then you can put this text in a file called `build.sbt`
+
+{{{
+scalaVersion := "2.12.6"
+libraryDependencies += "se.lth.cs" %% "introprog" % "1.0.0"
+}}}
+
+When you run `sbt` in terminal the introprog lib is automatically downloaded and made available on your classpath.
+You can do things like:
+
+{{{
+> sbt
+sbt> console
+scala> val w = new introprog.PixelWindow()
+scala> w.fill(100,100,100,100,java.awt.Color.red)
+}}}