Add collector documentation

collectors/cpufreqMetric.md

@@ -0,0 +1,11 @@
+## `cpufreq_cpuinfo` collector
+```json
+  "cpufreq": {
+    "exclude_metrics": []
+  }
+```
+
+The `cpufreq` collector reads the clock frequency from `/sys/devices/system/cpu/cpu*/cpufreq` and outputs a handful **cpu** metrics.
+
+Metrics:
+* `cpufreq`