README: add letsencrypt configuration

docs/letsencrypt_photos.crans.org

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+# To generate the certificate, please use the following command
+# certbot --config /etc/letsencrypt/conf.d/photos.crans.org.ini certonly
+
+# Use a 4096 bit RSA key instead of 2048
+rsa-key-size = 4096
+
+# Uncomment and update to register with the specified e-mail address
+email = photos@crans.org
+
+# Uncomment to use a text interface instead of ncurses
+text = True
+
+# Yes I want to sell my soul and my guinea pig.
+# UNCOMMENT ME when deploying this file to agree to terms
+#agree-tos = True
+
+# Use NGINX challenge
+authenticator = nginx
+
+cert-name = photos.crans.org
+domains = photos.crans.org