Update README.md

adding mandatory AUTH0_PROXY_SERVER_URL in ReadMe

Author: sachin-maheshwari

README.md

@@ -18,7 +18,7 @@ const busApi = require('tc-bus-api-wrapper')
 const busApiClient = busApi(_.pick(config,
       ['AUTH0_URL', 'AUTH0_AUDIENCE', 'TOKEN_CACHE_TIME',
         'AUTH0_CLIENT_ID', 'AUTH0_CLIENT_SECRET', 'BUSAPI_URL',
-        'KAFKA_ERROR_TOPIC']))
+        'KAFKA_ERROR_TOPIC', 'AUTH0_PROXY_SERVER_URL']))
 ```
 
 **Configuration / Environment variables:**
@@ -107,4 +107,4 @@ To run tests with coverage report, execute the command
 
 ```
 npm run cov
-```
+```