Update documentation to include setting authToken via DSM_AUTH_TOKEN environment variable

[wrightnathan]

I was trying to find a way to publish to DSM during my app's automated build process without exposing the authToken in plain text. I found the following code which appears to allow setting it via the DSM_AUTH_TOKEN environment variable:

dsm-storybook-cli/src/cli/build-configuration.js

Line 72 in fb6c9e4

const authToken = process.env.DSM_AUTH_TOKEN;

I couldn't find any mention of this in the configuration documentation. It would be nice to add it there.

[YoungElPaso]

Ah! Thanks so much for filing this issue, I didn't find that information either and also require it - Google led me here and only here! Instructions saying to paste the token right into the dsmrc file doesn't seem like the most secure way to handle this.

Be great if this was documented.