Added dump example

CONFIG.md

@@ -81,6 +81,19 @@ After this, we get:
             1 <chunk> = "STARDUST"
             2 <varint> = 100
 
+If you would like to a dump a field to a file, perhaps because it contains bytes that are a file, then use `dump` as follows:
+
+~~~ python
+types = {
+  "root": {
+    1: "dump",
+    2: "dump",
+  },
+}
+~~~
+
+This dumps field 1 and field 2 to the files `dump.0` and `dump.1`. 
+
 Here's the list of native types supported:
 
 Wire type | Type | Description

lib/types.py

@@ -240,9 +240,8 @@ def parse_enum(self, x, type):
     def parse_dump(self, file, type):
         chunk = file.read()
         filename = self.dump_prefix + str(self.dump_index)
-        file = open(filename, "w")
-        file.write(chunk)
-        file.close()
+        with open(filename, "wb") as f:
+            f.write(chunk)
         self.dump_index += 1
         return "%d bytes written to %s" % (len(chunk), filename)