Add Let's Encrypt details in README.md

[cupracer]

# certbot certonly --webroot -w /srv/www/htdocs -d $MYHOSTNAME
Saving debug log to /var/log/certbot/letsencrypt.log
Enter email address (used for urgent renewal and security notices) (Enter 'c' to
cancel):[email protected]
Starting new HTTPS connection (1): acme-v01.api.letsencrypt.org

-------------------------------------------------------------------------------
Please read the Terms of Service at
https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf. You must
agree in order to register with the ACME server at
https://acme-v01.api.letsencrypt.org/directory
-------------------------------------------------------------------------------
(A)gree/(C)ancel: a

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Would you be willing to share your email address with the Electronic Frontier
Foundation, a founding partner of the Let's Encrypt project and the non-profit
organization that develops Certbot? We'd like to send you email about EFF and
our work to encrypt the web, protect its users and defend digital rights.
-------------------------------------------------------------------------------
(Y)es/(N)o: n
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for mail.XXX.de
Using the webroot path /srv/www/htdocs for all unmatched domains.
Waiting for verification...
Cleaning up challenges

IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at
   /etc/certbot/live/mail.XXX.de/fullchain.pem. Your cert will expire
   on XXXX-XX-XX. To obtain a new or tweaked version of this
   certificate in the future, simply run certbot again. To
   non-interactively renew *all* of your certificates, run "certbot
   renew"
 - Your account credentials have been saved in your Certbot
   configuration directory at /etc/certbot. You should make a secure
   backup of this folder now. This configuration directory will also
   contain certificates and private keys obtained by Certbot so making
   regular backups of this folder is ideal.
 - If you like Certbot, please consider supporting our work by:

   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
   Donating to EFF:                    https://eff.org/donate-le