Added README to PCM collector plugin

plugin/collector/pulse-collector-pcm/README.md

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+## Pulse PCM Collector Plugin
+
+# Description
+Collect metrics from PCM (Intel Performance Counter Monitor)
+
+# Assumptions
+User is required to have PCM installed in system. It can be downloaded from [Intel PCM](http://www.intel.com/software/pcm) site.
+
+# Suggestions
+* Currently, Ubuntu 14.04 users have to manually compile it and add it to $PATH or $PULSE_PCM_PATH to be able to use it.
+* pulsed has to be running with root privileges, for ability to collect data from PCM.
+* To be able, to use PCM, [NMI watchdog](https://en.wikipedia.org/wiki/Non-maskable_interrupt) needs to be disabled. There are two ways to do this:
+ * at running time: `echo 0 > /proc/sys/kernel/nmi_watchdog`
+ * permanently: `echo 'kernel.nmi_watchdog=0' >> /etc/sysctl.conf`