Add AUTHORS.md and its update script

AUTHORS.md

+The following people have contributed to Simple Bandwidth Scanner.
+Thank you for helping make Tor better.
+
+* Matt Traudt
+* juga0
+
+*Last updated: 2018-04-02 on 018aba9*

scripts/maint/update-authors

+#!/usr/bin/env bash
+
+# Add <%aE> to the format string to get emails too
+AUTHORS=$(git log --format='%aN' | sort -u |
+    while read LINE
+    do
+        printf "* $LINE\n"
+    done
+)
+DATE=$(date +%Y-%m-%d)
+COMMIT=$(git log -n 1 --format='%h')
+
+cat << EOF
+The following people have contributed to Simple Bandwidth Scanner.
+Thank you for helping make Tor better.
+
+$AUTHORS
+
+*Last updated: $DATE on $COMMIT*
+EOF