Camera: Metadata XML cleanup, add README

Change-Id: Ib53757029becbd4148867a144ba3725c4f9e5d82
diff --git a/camera/docs/metadata_model.py b/camera/docs/metadata_model.py
index b6215ae..e916c69 100644
--- a/camera/docs/metadata_model.py
+++ b/camera/docs/metadata_model.py
@@ -248,7 +248,7 @@
     """
     tag_ids = [tg.name for tg in self.tags if tg.name == tag]
     if not tag_ids:
-      self._tags.append(Tag(tag, self))
+      self._tags.append(Tag(tag, self, description))
 
   def insert_entry(self, entry):
     """