Quoth RFC1036bis:

    If  a  poster or posting agent does append a signature to an
    article, the signature SHOULD be preceded with  a  delimiter
    line  containing  (only)  two hyphens (ASCII 45) followed by
    one blank (ASCII  32).

Note that this isn't a standard in any formal sense, and RFC1036 is a
Usenet news thing.  However, it has been the done thing since, well,
forever.  It works.