add small documentation for the posterity

Author: umbynos

Diff for: certs/README.md

@@ -0,0 +1,9 @@
+# Certificates
+
+The certificates in this directory have to be mirrored from the ones present in the
+[nina-fw repos](https://github.com/arduino/nina-fw/blob/master/data/roots.pem).
+
+They are there too because if the user decides to upload a new cert, the partition inside the NINA module containing the
+certs gets wiped-out, so it's required to upload the cert with a single command-line containing all the cert the user
+wants on the board. For further details see the
+[docs](https://arduino.github.io/arduino-fwuploader/latest/usage/#certificates)