Found under Documentation > Configure > Custom Domain

We will automatically generate an SSL certificate on your behalf using Let’s Encrypt

Let's Encrypt is a certificate authority which can be used to generate totally free X.509 (SSL/TLS) certificates. All you need to do to benefit from this is to setup a CNAME to our SSL server, "ssl.readmessl.com".

To allow users to not use SSL, click General Settings > Custom Domain > Advanced Options and uncheck the Force SSL box.

If you decide to have a CAA record set, make sure you use "letsencrypt.org" instead of "letsencrypt.com".