documentation: describe role of fullchain.pem

[vielmetti]

The documentation doesn't mention what fullchain.pem is, but if you run it naively and without configuring anything all, it errors out:

Eds-MacBook-Pro:~ ed$ sudo go/bin/ssl-tester --help
2019/02/02 01:30:25 open /etc/ssl-tester/fullchain.pem: no such file or directory

Perhaps a brief paragraph somewhere as to what that file is and how it got there? I'm looking at https://community.letsencrypt.org/t/generating-cert-pem-chain-pem-and-fullchain-pem-from-order-certificate/78376 help sort that out.