diff --git a/third_party/go/README b/third_party/go/README
index cbd26a204..26e3ea3e5 100644
--- a/third_party/go/README
+++ b/third_party/go/README
@@ -1,3 +1,14 @@
-This directory contains portions of the standard library imported from tip to
-take advantage of new features and bug fixes before they're available in the
-stable branch.
+This directory contains code from the Go standard library either
+imported from tip (to take advantage of new features and bug fixes
+before they're available in the stable branch), or modified in ways
+that Camlistore needs.
+
+A summary:
+
+* pkg/image/jpeg: performance improvements going into Go 1.4
+  such as https://codereview.appspot.com/109050045/
+
+* pkg/archive/zip: new writing LastDataOffset accessor needed for
+  blobpacked
+
+See the git history for more details.
diff --git a/third_party/go/pkg/archive/zip/writer.go b/third_party/go/pkg/archive/zip/writer.go
index 6c9800a78..3dc5ea308 100644
--- a/third_party/go/pkg/archive/zip/writer.go
+++ b/third_party/go/pkg/archive/zip/writer.go
@@ -22,6 +22,12 @@ type Writer struct {
 	dir    []*header
 	last   *fileWriter
 	closed bool
+
+	// LastDataOffset is the offset from the beginning of the zip
+	// file where the most recently created file's data will be
+	// (possibly be compressed). This offset is past the zip
+	// header.
+	LastDataOffset int64
 }
 
 type header struct {
@@ -219,6 +225,7 @@ func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 		return nil, err
 	}
 
+	w.LastDataOffset = w.cw.count
 	w.last = fw
 	return fw, nil
 }