Update README to indicate port 80 caeveat.

Author: ollien

README.md

@@ -46,6 +46,8 @@ volumes:
 
 Launch that docker-compose file, and you're good to go; `certbot` will automatically request an SSL certificate for any `nginx` sites that look for SSL certificates in `/etc/letsencrypt/live`, and will automatically renew them over time.
 
+Note: using a `server` block that listens on port 80 may cause issues with renewal. This container will already handle forwarding to port 443, so they are unnecessary.
+
 ## Templating
 
 You may wish to template your configurations, e.g. passing in a hostname so as to be able to run multiple identical copies of this container; one per website.  The docker container will use [`envsubst`](https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html) to template all mounted user configs with a user-provided list of environment variables.  Example: