Document support for secrets in README

[JacobSnyder]

I try to avoid putting secrets into environment variables wherever it's supported. I noticed that the source mention Docker secrets instead of the environment variables specified in this README.

I tried it and it works! Here's the relevant snippets I found from the logs:

OZW_NETWORK_KEY

$ cat openzwave/logs/ozwdaemon/current | grep "Network Key"
2021-01-25 01:29:29.328442275  [20210125 1:29:29.328 UTC] [ozw.daemon] [warning]: Network Key Specified in Enviroment is Invalid
2021-01-25 01:29:29.328790712  [20210125 1:29:29.328 UTC] [ozw.daemon] [info]: Network Key From File is Valid - Using File
2021-01-25 01:29:29.328914045  [20210125 1:29:29.328 UTC] [ozw.daemon] [info]: We Have what appears to be a valid Network Key - Passing to OZW
2021-01-25 01:29:29.420419805  [20210125 1:29:29.420 UTC] [ozw.library] [info]: Info - Node: 0 Setting Up Provided Network Key for Secure Communications

OZW_AUTH_KEY
The auth key doesn't have as much logging as the network key, but it does have some.

$ cat openzwave/logs/ozwdaemon/current | grep "Authentication Key"
2021-01-25 01:29:29.329138524  [20210125 1:29:29.328 UTC] [ozw.daemon] [info]: Using Remote Authentication Key

MQTT_PASSWORD
There isn't any "positive" logging that confirms that the MQTT password has been picked up, but all of the "negative" logging that indicates problems includes the phrase "MQTT_PASSWORD" and I don't have any instances of that in my logs. And, my MQTT broker logged a successful connection from OZW.

$ cat openzwave/logs/ozwdaemon/current | grep "MQTT_PASSWORD"
<nothing>

Thanks!