added encoded API key placeholder for updated api_key elasticsearch authentication.

Author: SamoraHunter

credentials.py

@@ -8,6 +8,7 @@
 username = None
 password = None
 
+api_key = None # Encoded api key issued by your cogstack administrator.
 
 # NLM authentication
 # The UMLS REST API requires a UMLS account for the authentication described below.