Limit node-exporter scope, deprecated collectors

CONTRIBUTING.md

@@ -16,3 +16,26 @@ Prometheus uses GitHub to manage reviews of pull requests.
   and the _Formatting and style_ section of Peter Bourgon's [Go: Best
   Practices for Production
   Environments](http://peter.bourgon.org/go-in-production/#formatting-and-style).
+
+
+## Collector Implementation Guidelines
+
+The Node Exporter is not a general monitoring agent. Its sole purpose is to
+expose machine metrics, as oppose to service metrics, with the only exception
+being the textfile collector.
+
+The metrics should not get transformed in a way that is hardware specific and
+would require maintaining any form of vendor based mappings or conditions. If
+for example a proc file contains the magic number 42 as some identifier, the
+Node Exporter should expose it as it is and not keep a mapping in code to make
+this human readable. Instead, the textfile collector can be used to add a static
+metric which can be joined with the metrics exposed by the exporter to get human
+readable identifier.
+
+A Collector may only read `/proc` or `/sys` files, use system calls or local
+sockets to retrieve metrics. It may not require root privileges. Running
+external commands is not allowed for performance and reliability reasons. Use a
+dedicated exporter instead or gather the metrics via the textfile collector.
+
+The Node Exporter tries to support the most common machine metrics. For more
+exotic metrics, use the textfile collector or a dedicated Exporter.