Add template for example modules.

docs/examples/template.yin

+<?xml version="1.0" encoding="utf-8"?>
+<module name="example-n"
+        xmlns="urn:ietf:params:xml:ns:yang:yin:1"
+        xmlns:test="http://example.com/example-n">
+  <yang-version value="1.1"/>
+  <namespace uri="http://example.com/example-n"/>
+  <prefix value="exn"/>
+</module>