nfd-status: Define format and implement --xml option

The ether URI format "ether://01:00:5e:00:17:aa" violates
the URI specification, so it has been changed to
ether://[01:00:5e:00:17:aa] in this commit.

refs: #1438

Change-Id: I1218374b75f919a5e1ab158bbab2e217e64aced3
diff --git a/docs/schema.rst b/docs/schema.rst
new file mode 100644
index 0000000..08e40b7
--- /dev/null
+++ b/docs/schema.rst
@@ -0,0 +1,10 @@
+NFD Status XML Schema
+=====================
+
+The NFD status XML schema describes the structure of the NFD status XML
+document generated by command ``nfd-status``.
+
+Users can use this schema to validate the generated XML document.
+
+.. literalinclude:: _static/nfd-status.xsd
+   :language: xml