Update Secrets documentation

[TimMoore]

https://developer.lightbend.com/docs/lightbend-orchestration/current/features/secrets.html

With the deprecation of SecretReader in lightbend/reactive-lib#118, the documentation needs to be updated to show how to read secrets from the filesystem.

Like the existing implementation, the code example should be non-blocking so that people won't get tripped up when using it from Actors, async Play actions, Lagom, etc.

There should be both Java and Scala examples.

[TimMoore]

We could also remove "This feature will be made available for DC/OS at a later date."

[TimMoore]

Needed for the 1.7.0 release.

[TimMoore]

Sorry I put this in the wrong repo. Moving to lightbend-orchestration-docs...