chore: add README for slow subscriptions

apps/emqx_slow_subs/README.md

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+# EMQX Slow Subscriptions
+
+This application can calculate the latency (time spent) of the message to be processed and transmitted since it arrives at EMQX.
+
+If the latency exceeds a specified threshold, this application will add the subscriber and topic information to a slow subscriptions list or update the existing record.
+
+# How To Use
+
+You can add the below section into `emqx.conf` to enable this application
+
+
+```yaml
+slow_subs {
+  enable = true
+  threshold = "500ms"
+  expire_interval = "300s"
+  top_k_num = 10
+  stats_type = whole
+}
+```
+
+**threshold**: Latency threshold for statistics, only messages with latency exceeding this value will be collected.
+
+Minimum value: 100ms  
+Default value: 500ms
+
+**expire_interval**: Eviction time of the record, will start the counting since the creation of the record, and the records that are not triggered again within this specified period will be removed from the rank list.
+
+Default: 300s
+
+**top_k_num**: Maximum number of records in the slow subscription statistics record table. 
+
+Maximum value: 1,000  
+Default value: 10
+
+**stats_type**: Calculation methods of the latency, which are
+- **whole**: From the time the message arrives at EMQX until the message transmission completes
+- **internal**: From when the message arrives at EMQX until when EMQX starts delivering the message
+- **response**: From the time EMQX starts delivering the message until the message transmission completes
+
+Default value: whole