LogDog: Add Butler stream server package.

client/logdog/butlerlib/streamproto/magic.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package streamproto
 
 var (
         // ProtocolFrameHeaderMagic is the number at the beginning of streams that
         // identifies the stream handshake version.
+        //
+        // This serves two purposes:
+        //   - To disambiguate a Butler stream from some happenstance string of bytes
+        //     (which probably won't start with these characters).
+        //   - To allow an upgrade to the wire format, if one is ever needed. e.g.,
+        //     a switch to something other than recordio/JSON.
         ProtocolFrameHeaderMagic = []byte("BTLR1\x1E")
 )