Update instructions

[jwillmer]

I find it hard to follow your instructions. I used the example MQTT_x509_bin.ino and found out that xxd is a Linux command that transforms the converted DER file to byte code. I downloaded xxd for Windows and did this. Now I tested the code and i get a -4 error back from AWS. I also found the AWSconnectivity diagnose website but there I need PEM files.

Could you update the documentation to make it easier to follow? Maybe I can then figure out what I did wrong in order to get a -4 error. Do you know there I can look the error codes up?

[chegewara]

Thanks, you are life saver.