Point to WMI exporter (#431)

There's a slow trickle of people trying to use the node exporter on windows, so point them in the right direction.

README.md

@@ -4,9 +4,11 @@
 [![Docker Repository on Quay](https://quay.io/repository/prometheus/node-exporter/status)][quay]
 [![Docker Pulls](https://img.shields.io/docker/pulls/prom/node-exporter.svg?maxAge=604800)][hub]
 
-Prometheus exporter for hardware and OS metrics exposed by the kernel, written
+Prometheus exporter for hardware and OS metrics exposed by \*NIX kernels, written
 in Go with pluggable metric collectors.
 
+The [WMI exporter](https://github.com/martinlindhe/wmi_exporter) is recommended for Windows users. 
+
 ## Collectors
 
 There is varying support for collectors on each operating system. The tables