expand the documentation regarding the certificates flashing (#129)

* expand the documentation regarding the certificates flashing

* Apply suggestions from code review

Co-authored-by: per1234 <accounts@perglass.com>

Co-authored-by: per1234 <accounts@perglass.com>

docs/usage.md

@@ -74,7 +74,14 @@ The tool offers also the ability to flash SSL certificates to a module:
 ./arduino-fwuploader certificates flash -b arduino:samd:nano_33_iot" -a COM10 -u arduino.cc:443 -u google.com:443
 ```
 
-or you can specify a path to a file with `-f` instead of the URL of the certificate
+or you can specify a path to a file in `.der` format with `-f` instead of the URL of the certificate
+
+Due to a limitation on the handling of certs in the NINA modules, when `certificates flash` command is launched, all the
+previous certificates are going to be erased. To overcome this it's required to upload them all together:
+
+```
+./arduino-fwuploader certificates flash --url arduino.cc:443,google.com:443 --file ~/Downloads/my_self_signed_.der -b arduino:mbed_nano:nanorp2040connect -a /dev/ttyACM0
+```
 
 ### Command line options