Add xxx-single-guard-node.txt .
Nick Hopper changes

Acked-by: George Kadianakis <desnacked@riseup.net>
Add Nick's name in the authors and delete some trailing whitespace.
Clarify that 0 is the only reserved circID value

Fixes 11565.
Merge remote-tracking branch 'asn/single_guard'
Add xxx-single-guard-node as proposal 236.