documentation: must run as root

[vielmetti]

If you get permission denied when running this, note that it binds to a low port, and thus on most systems should run as root.

Eds-MacBook-Pro:~ ed$ go/bin/ssl-tester 
2019/02/02 01:29:56 listen tcp :443: bind: permission denied
Eds-MacBook-Pro:~ ed$ sudo go/bin/ssl-tester 
Password:
2019/02/02 01:30:10 open /etc/ssl-tester/fullchain.pem: no such file or directory