Added default config to file output pugin's README (#2426)

Author: theist

plugins/outputs/file/README.md

@@ -1 +1,16 @@
 # file Output Plugin
+
+This plugin writes telegraf metrics to files
+
+### Configuration
+```
+[[outputs.file]]
+  ## Files to write to, "stdout" is a specially handled file.
+  files = ["stdout", "/tmp/metrics.out"]
+
+  ## Data format to output.
+  ## Each data format has it's own unique set of configuration options, read
+  ## more about them here:
+  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md
+  data_format = "influx"
+```