Document Secrets

[Huskydog9988]

Is your feature request related to a problem? Please describe.
Document how to tell vault unseal to use a file, as per #6. I'd also like to see documention of the existance of the env vars, unless i'd dug through the code I wouldn't have found them.

Describe the solution you'd like
Just include the documentation in the readme.

Describe alternatives you've considered
You could just not.

Explain any additional use-cases

Additional context