Merge pull request #85 from dn-m/docs

Add basic README

README.md

@@ -1,5 +1,15 @@
 # NotationModel
 
+![Swift](https://img.shields.io/badge/Swift-4.2-brightgreen.svg)
 [![Build Status](https://travis-ci.org/dn-m/NotationModel.svg?branch=master)](https://travis-ci.org/dn-m/NotationModel)
 
-Model of musical notation
+The `NotationModel` package defines structures for describing how musical information can be graphically represented, while being agnostic to the concrete method of graphical representation.
+
+The `NotationModel` package contains several modules:
+
+- [`SpelledPitch`](https://github.com/dn-m/NotationModel/tree/master/Sources/SpelledPitch)
+- [`PitchSpeller`](https://github.com/dn-m/NotationModel/tree/master/Sources/PitchSpeller)
+- [`SpelledRhythm`](https://github.com/dn-m/NotationModel/tree/master/Sources/SpelledRhythm)
+- [`RhythmBeamer`](https://github.com/dn-m/NotationModel/tree/master/Sources/SpelledPitch)
+- [`PlotModel`](https://github.com/dn-m/NotationModel/tree/master/Sources/PlotModel)
+- [`StaffModel`](https://github.com/dn-m/NotationModel/tree/master/Sources/StaffModel)