add usage instructions

README.md

@@ -1,7 +1,25 @@
 # Mittsu: 3MF
-3MF support for Mittsu
+3MF support for [Mittsu](https://github.com/danini-the-panini/mittsu).
 
+## Installation
 
-# About
+Just install:
+
+`bundle add mittsu-3mf`
+
+Then require in your code:
+
+`require 'mittsu/3mf'`
+
+## Usage
+
+Currently this gem just includes an exporter. Loading 3MF files is "coming soon".
+
+```
+exporter = Mittsu::ThreeMFExporter.new
+exporter.export(object, "output.3mf")
+```
+
+## About
 
 This code was originally written for [Manyfold](https://manyfold.app), supported by funding from [NLNet](https://nlnet.nl) and [NGI Zero](https://ngi.eu/ngi-projects/ngi-zero/).