This commit includes:
1) Make descriptiuon fields mandatory
2) Make some minor changes to comply to IETF
3) Add a few test cases (more to come)

Change-Id: Id871b2015ddb72c72a03c9fd5d0e08461fe67398
diff --git a/experiments/proto2yang/addressbook.proto b/experiments/proto2yang/addressbook.proto
new file mode 100644
index 0000000..fc1a10f
--- /dev/null
+++ b/experiments/proto2yang/addressbook.proto
@@ -0,0 +1,34 @@
+// See README.txt for information and build instructions.
+
+syntax = "proto3";
+
+package tutorial;
+
+option java_package = "com.example.tutorial";
+option java_outer_classname = "AddressBookProtos";
+option csharp_namespace = "Google.Protobuf.Examples.AddressBook";
+
+message Person {
+  string name = 1;
+  int32 id = 2;        // Unique ID number for this person.
+  string email = 3;
+
+  enum PhoneType {
+    MOBILE = 0;
+    HOME = 1;
+    WORK = 2;
+  }
+
+  message PhoneNumber {
+    string number = 1;
+    PhoneType type = 2;
+  }
+
+  repeated PhoneNumber phones = 4;
+  repeated string khen = 5;
+}
+
+// Our address book file is just one of these.
+message AddressBook {
+  repeated Person people = 1;
+}