Comment about self-signed certs and CAcert in the documentation
From https://github.com/pump-io/pump.io/issues/1151:
As per #786, looks desirable to explain some details about SSL certs in the documentation.
@dper's Suggestion:
If you are looking for a free SSL certificate, please consider Let's Encrypt. Self-signed certificates and certificates signed by less-trusted providers like CACert appear to work when you're installing, but later your server may have trouble communicating with other pump servers.