Add VoIP certificate instruction to exclude the cert header (bag attributes) (#537)

Author: bobiechen-twilio

README.md

@@ -156,6 +156,7 @@ Once you have generated the VoIP Services Certificate, you will need to provide
 Export your VoIP Service Certificate as a `.p12` file from *Keychain Access* and extract the certificate and private key from the `.p12` file using the `openssl` command. 
 
  $ openssl pkcs12 -in PATH_TO_YOUR_P12 -nokeys -out cert.pem -nodes
+ $ openssl x509 -in cert.pem -out cert.pem
  $ openssl pkcs12 -in PATH_TO_YOUR_P12 -nocerts -out key.pem -nodes
  $ openssl rsa -in key.pem -out key.pem