Documentation/security-bugs: Explain why plain text is preferred

The security contact list gets regular reports contained in archive
attachments. This tends to add some back-and-forth delay in dealing with
security reports since we have to ask for plain text, etc.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/202007091110.205DC6A9@keescook


Signed-off-by: Jonathan Corbet <corbet@lwn.net>