filtering header draft rev 35

FYI The miller-et-al filter header document is now at revision 35,
and includes a few more sections. It's available at

 http://millenix.zemos.net/asrg-filtering/

Changes from rev 31 are a new "authentication" section, and a bigger
list of field values for the header line. The text is still "rough".

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Laird Breyer.