Add Let's Encrypt instructions to README

README.md

 
 Let's Encrypt
 -------------
+If you want to add more domains after deployment, you can run this command manually from HAProxy docker instance (see [this](https://serversforhackers.com/c/letsencrypt-with-haproxy)):
+```
+sudo certbot certonly --standalone -d nextcloud.example.com -d gogs.example.com \
+    --non-interactive --agree-tos --email admin@example.com \
+    --http-01-port=8888
+```
+
 Notice that when updating your certificate, you will need to restart haproxy container, due to [this issue](https://stackoverflow.com/a/50480260/1937418)
 
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