readme: Add command to renew Let's Encrypt certificate

...and restart Haproxy

README.md

 
 You can use the following script to add users if you have previously created `ldif` files:
 
-    ./add_users <your-stack-name>
+    ./add_users.sh <your-stack-name>
 
 
 MariaDB
 
 Notice that when updating your certificate, you will need to restart haproxy container, due to [this issue](https://stackoverflow.com/a/50480260/1937418)
 
+You can do it with:
+
+```
+docker exec -ti dhs_haproxy_1 /etc/cron.daily/letsencrypt && docker stop dhs_haproxy_1  &&  docker-compose -p dhs up -d haproxy
+```
+
 Own registry
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