Add example content for a secret file

[misterjoonas]

I could not remember how the secret file should be formatted by heart, so an example is good to be there.

[tormi]

Yaml format requirement is already covered in the same README section: The silta/silta.secret file is a YAML file...

The particular content depends on data which needs to be encrypted, f.ex this is also valid:

smtp:
  password: 'password'

We can add a reference to https://github.com/wunderio/charts/blob/master/drupal/values.yaml if you feel so.

[misterjoonas]

I'm just trying to make it as easy as possible, I think most of the time developers need to override php environment variables and that example helps with that. Without understanding how the values.yml file or Silta configurations work in general. And help people like me who need to add secrets a few times per year or so...