README.md: add max_shards: 100 to the recommended Prometheus config

Prometheus establishes a connection per shard in remote_write config.
By default it establishes up to 1000 connections to remote storage (max_shards: 1000).
This is quite big, so set `max_shards: 100` in the recommmended Prometheus config.

README.md

@@ -152,6 +152,7 @@ remote_write:
   - url: http://<victoriametrics-addr>:8428/api/v1/write
     queue_config:
       max_samples_per_send: 10000
+      max_shards: 100
 ```
 
 Substitute `<victoriametrics-addr>` with the hostname or IP address of VictoriaMetrics.