add NATS input docs
+When using NATS as input, `gnmic` consumes data from a specific NATS subject in `event` or `proto` format.
+
+Multiple consumers can be created per `gnmic` instance (`num-workers`).
+All the workers join the same [NATS queue group](https://docs.nats.io/nats-concepts/queue) (`queue`) in order to load share the messages between the workers.
+
+Multiple instances of `gnmic` with the same NATS input can be used to effectively consume the exported messages in parallel
+
+The NATS input will export the received messages to the list of outputs configured under its `outputs` section.
+
+```yaml
+inputs:
+ input1:
+ # string, required, specifies the type of input
+ type: nats
+ # string, NATS consumer name.
+ # If left empty, a name in `gnmic-$uuid` format is generated
+ Name:
+ # string, comma separated NATS servers addresses
+ address: localhost:4222
+ # The subject name gnmic NATS consumers subscribe to.
+ subject: telemetry
+ # subscribe queue group all gnmic NATS input workers join,
+ # so that NATS server can load share the messages between them.
+ queue:
+ # string, NATS username
+ username:
+ # string, NATS password
+ password:
+ # duration, wait time before reconnection attempts
+ connect-time-wait: 2s
+ # string, consumed message expected format, one of: proto, event
+ format: event
+ # bool, enables extra logging
+ debug: false
+ # integer, number of nats consumers to be created
+ num-workers: 1
+ # []string, list of named outputs to export data to.
+ # Must be configured under root level `outputs` section
+ outputs:
+```
+
