README.md in vagrant-libvirt-0.0.2

- old
+ new

@@ -10,5 +10,14 @@
 ```
 
 This box works by using Vagrant's built-in Vagrantfile merging to setup
 defaults for Libvirt. These defaults can easily be overwritten by higher-level
 Vagrantfiles (such as project root Vagrantfiles).
+
+## Box Metadata
+
+Libvirt box should define at least three data fields in `metadata.json` file.
+
+* provider - Provider name is libvirt.
+* format - Currently supported format is qcow2.
+* virtual_size - Virtual size of image in GBytes.
+