Add websocket usage in Polygon plugin

Polygon's preferred method of subscribing to streaming services is no longer NATS.  Instead, users should use websockets.  The old readme did not detail how to use websockets, so I added `ws_servers` to the table and updated the example.

contrib/polygon/README.md

@@ -1,10 +1,10 @@
 # Polygon Data Fetcher
 
 This module builds a MarketStore background worker which fetches historical
-price data of US stocks from [Polygon's API](https://polygon.io/).  It runs
+price data of US stocks from [Polygon's WebSockets Clusters](https://polygon.io/sockets).  It runs
 as a goroutine behind the MarketStore process and keeps writing to the disk.
 The module uses both the HTTP interface (to backfill from MarketStore's last
-written record), and the NATS streaming interface (to receive real-time updates).
+written record), and websockets streaming interface (to receive real-time updates).
 
 ## Configuration
 polygon.so comes with the server by default, so you can simply configure it
@@ -17,6 +17,7 @@ data_types | slice of strings | none | List of data types (bars, quotes, trades)
 api_key | string | none | Your polygon api key
 base_url | string | none | The URL to use in the HTTP client
 nats_servers | string | Comma separated list of nats servers to connect to
+ws_servers | string | Comma separated list of websocket servers to connect to
 symbols | slice of strings | none | The symbols to retrieve chart bars for
 
 ### Example
@@ -25,10 +26,10 @@ Add the following to your config file:
 bgworkers:
   - module: polygon.so
     config:
-        api_key: your_api_key
-        base_url: https://api.polygon.io
-        nats_servers: nats://nats1.polygon.io:30401, nats://nats2.polygon.io:30402, nats://nats3.polygon.io:30403
-        symbols:
-          - AAPL
-          - SPY
+      api_key: your_api_key
+      ws_servers: wss://alpaca.socket.polygon.io
+      data_types: ["bars"]
+      symbols:
+        - AAPL
+        - SPY
 ```