Add code comment

Adding a code comment that explains setting MD5 Sum field.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/storage/driver/gcs/gcs.go

@@ -426,6 +426,10 @@ func (d *driver) putContent(ctx context.Context, obj *storage.ObjectHandle, cont
 	if _, err := bytes.NewReader(content).WriteTo(wc); err != nil {
 		return err
 	}
+	// NOTE(milosgajdos): Apparently it's posisble to to upload 0-byte content to GCS.
+	// Setting MD5 on the Writer helps to prevent presisting that data.
+	// If set, the uploaded data is rejected if its MD5 hash does not match this field.
+	// See: https://pkg.go.dev/cloud.google.com/go/storage#ObjectAttrs
 	h := md5.New()
 	h.Write(content)
 	wc.MD5 = h.Sum(nil)