Document format of privateKey config

It would be nice to document the expected format of the private key. I need use get an IMS token from a serverless action, and passing in all secrets as strings to the action. I can't seem to get the private key in the correct format for this library to accept it.

When using it w/ my private key i get the following error: `error: error:0909006C:PEM routines:get_name:no start line` .. which i assume is an issue w/ my private key format.

Start by reproducing the PEM error with a serverless action's string secret and inspect how the library accepts its privateKey input. Document the accepted key format and the handling needed for the example; done means a user can format the secret and obtain an IMS token without the no-start-line error.

Written by the indexing model from the issue text.