Update README and receiver-specific pages

receivers/natsReceiver.md

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+## `nats` receiver
+
+The `nats` receiver can be used receive metrics from the NATS network. The `nats` receiver subscribes to the topic `database` and listens on `address` and `port` for metrics in the InfluxDB line protocol.
+
+### Configuration structure
+
+```json
+{
+  "<name>": {
+    "type": "nats",
+    "address" : "nats-server.example.org",
+    "port" : "4222",
+    "subject" : "subject"
+  }
+}
+```
+
+- `type`: makes the receiver a `nats` receiver
+- `address`: Address of the NATS control server
+- `port`: Port of the NATS control server
+- `subject`: Subscribes to this subject and receive metrics