Explicitly state precedence of config sources in docs (#1439)

I was recently looking into the order in which oauth2-proxy evaluates it configuration options from the various sources.
I think this will also be helpful for other users.
Since oauth2-proxy is using viper, the order of configuration sources is as follows [1]:
> Viper uses the following precedence order. Each item takes precedence over the item below it:
>
>    explicit call to Set
>    flag
>    env
>    config
>    key/value store
>    default

[1] https://github.com/spf13/viper/blob/master/README.md#why-viper

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>