Add public local bundle writer

pkg/bundle/doc.go

@@ -3,6 +3,7 @@
 //
 // A source bundle is a local directory containing a manifest.json file and the
 // files listed by that manifest. This package owns the public manifest model,
-// digest calculation, path validation, manifest parsing, manifest building, and
-// local bundle validation used by Go producer applications.
+// digest calculation, path validation, manifest parsing, manifest building,
+// local bundle writing, and local bundle validation used by Go producer
+// applications.
 package bundle