Added instructions to change the secretKey

Author: raulraja

README.md

@@ -12,6 +12,17 @@ sbt run
 # Authentication
 
 The remote Scala eval uses [JWT](https://jwt.io/) to encode / decode tokens.
+The `secretKey` used for encoding/decoding is configurable as part of the service configuration in 
+`src/main/resources/application.conf`.
+
+Please change `secretKey` by overriding it or providing the `EVAL_SECRET_KEY` env var.
+
+```json
+eval.auth {
+  secretKey = "secretKey"	  
+  secretKey = ${?EVAL_SECRET_KEY}
+}
+```
 
 ## Generate an auth token