Document gunicorn loglevel setting

docs/server/source/server-reference/configuration.md

@@ -17,6 +17,7 @@ For convenience, here's a list of all the relevant environment variables (docume
 `BIGCHAINDB_DATABASE_NAME`<br>
 `BIGCHAINDB_DATABASE_REPLICASET`<br>
 `BIGCHAINDB_SERVER_BIND`<br>
+`BIGCHAINDB_SERVER_LOGLEVEL`<br>
 `BIGCHAINDB_SERVER_WORKERS`<br>
 `BIGCHAINDB_SERVER_THREADS`<br>
 `BIGCHAINDB_CONFIG_PATH`<br>
@@ -121,17 +122,22 @@ If you used `bigchaindb -y configure mongodb` to create a default local config f
 ```
 
 
-## server.bind, server.workers & server.threads
+## server.bind, server.loglevel, server.workers & server.threads
 
 These settings are for the [Gunicorn HTTP server](http://gunicorn.org/), which is used to serve the [HTTP client-server API](../drivers-clients/http-client-server-api.html).
 
 `server.bind` is where to bind the Gunicorn HTTP server socket. It's a string. It can be any valid value for [Gunicorn's bind setting](http://docs.gunicorn.org/en/stable/settings.html#bind). If you want to allow IPv4 connections from anyone, on port 9984, use '0.0.0.0:9984'. In a production setting, we recommend you use Gunicorn behind a reverse proxy server. If Gunicorn and the reverse proxy are running on the same machine, then use 'localhost:PORT' where PORT is _not_ 9984 (because the reverse proxy needs to listen on port 9984). Maybe use PORT=9983 in that case because we know 9983 isn't used. If Gunicorn and the reverse proxy are running on different machines, then use 'A.B.C.D:9984' where A.B.C.D is the IP address of the reverse proxy. There's [more information about deploying behind a reverse proxy in the Gunicorn documentation](http://docs.gunicorn.org/en/stable/deploy.html). (They call it a proxy.)
 
+`server.loglevel` sets the log level of Gunicorn's Error log outputs. See
+[Gunicorn's documentation](http://docs.gunicorn.org/en/latest/settings.html#loglevel)
+for more information.
+
 `server.workers` is [the number of worker processes](http://docs.gunicorn.org/en/stable/settings.html#workers) for handling requests. If `None` (the default), the value will be (cpu_count * 2 + 1). `server.threads` is [the number of threads-per-worker](http://docs.gunicorn.org/en/stable/settings.html#threads) for handling requests. If `None` (the default), the value will be (cpu_count * 2 + 1). The HTTP server will be able to handle `server.workers` * `server.threads` requests simultaneously.
 
 **Example using environment variables**
 ```text
 export BIGCHAINDB_SERVER_BIND=0.0.0.0:9984
+export BIGCHAINDB_SERVER_LOGLEVEL=debug
 export BIGCHAINDB_SERVER_WORKERS=5
 export BIGCHAINDB_SERVER_THREADS=5
 ```
@@ -140,6 +146,7 @@ export BIGCHAINDB_SERVER_THREADS=5
 ```js
 "server": {
     "bind": "0.0.0.0:9984",
+    "loglevel": "debug",
     "workers": 5,
     "threads": 5
 }
@@ -149,6 +156,7 @@ export BIGCHAINDB_SERVER_THREADS=5
 ```js
 "server": {
     "bind": "localhost:9984",
+    "loglevel": "info",
     "workers": null,
     "threads": null
 }