Lets Encrypt certificates are now trusted by all major browsers. This is a significant milestone since it means visitors to websites using these certificates can enjoy a secure browsing experience with no special configuration required. Web servers will need to be configured to serve the appropriate cross-signature certificate as part of the trust chain. The Let’s Encrypt client will handle this automatically. You can see an example of a server using a Lets Encryption certificate under a new cross-signed intermediate here.”]
Source: https://letsencrypt.org/2015/10/19/lets-encrypt-is-trusted.html