Improve README.

README.md

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 # Purpose
 
+I wanted to install a certificate generated by Let's encrypt on my brand new laser printer 
+(HP Laserjet MFJ 4302) for the integrated administration webserver.
+After research on the web, I discovered 
+
 # Arguments
 
+positional arguments:
+  {csr,pem}             command help
+    csr                 Create CSR
+    pem                 Install certificate
+
+options:
+  -h, --help            show this help message and exit
+  -d, --debug           Activate debug.
+  -c, --config CONFIG_FILENAME
+                        Configuration file.
+  -u, --user USERNAME   Username.
+  -p, --password [PASSWORD]
+                        Password.
+  -H, --host HOSTNAME   Hostname.
+  -n, --no-tls-verification
+                        Verify certificate validity.
+
+
 # How to use it ?
 
+To obtain a CSR:
+```
+./refresh-certificate.py  -c ./config.ini -n -p PASSWORD csr
+```
+
+To obtain a PEM certificate from Let's encrypt:
+```
+certbot certonly --webroot -w /var/www/letsencrypt/ --csr printer.csr
+```
+
+To install the PEM file on the printer:
+```
+./refresh-certificate.py  -c ./config.ini -n -p PASSWORD pem 
+```
+
+