add readme for procstat plugin

plugins/procstat/README.md

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+# Procstat plugin
+
+The procstat plugin can be used to monitor system resource usage by an
+individual process.
+
+Processes can be specified either by pid file or by executable name. Procstat
+plugin will use `pgrep` when executable name is provided to obtain the pid. Proctsta plugin will transmit IO, memory, cpu, file descriptor related measurements for every process specified. A prefix can be set to isolate individual process specific measurements.
+
+Example:
+
+```
+[procstat]
+
+[[procstat.specifications]]
+  exe = "influxd"
+  prefix = "influxd"
+
+[[procstat.specifications]]
+  pid_file = "/var/run/lxc/dnsmasq.pid"
+  prefix = "dnsmasq"
+```