[mailsync] Add SO credit for locking mechanism

mailsync

@@ -5,6 +5,7 @@ maildir=~/mail
 mailboxes=$(find "$maildir" -mindepth 1 -maxdepth 1 -type d ! -name cur ! -name tmp ! -name new ! -name .notmuch -printf '%f\n')
 
 LOCKFILE="/tmp/.mailsync.$USER.lock"
+# https://stackoverflow.com/questions/185451/quick-and-dirty-way-to-ensure-only-one-instance-of-a-shell-script-is-running-at
 # /!\ locking method is susceptible to race condition
 # Should be fine for cron job
 if [ -e "$LOCKFILE" ] && kill -0 "$(cat "$LOCKFILE")"; then