add REAME about TCP Connection plugin.

plugins/system/TCPCONN_README.md

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+Telegraf plugin: TCPCONN
+
+#### Description
+
+The TCPCONN plugin collects TCP connections state by using `lsof`. 
+
+Supported TCP Connection states are follows. 
+
+- established
+- syn_sent
+- syn_recv
+- fin_wait1
+- fin_wait2
+- time_wait
+- close
+- close_wait
+- last_ack
+- listen
+- closing
+- none
+
+
+# Measurements:
+### TCP Connections measurements:
+
+Meta:
+- units: counts
+
+Measurement names:
+- established
+- syn_sent
+- syn_recv
+- fin_wait1
+- fin_wait2
+- time_wait
+- close
+- close_wait
+- last_ack
+- listen
+- closing
+- none
+
+If there are no connection on the state, the metric is not counted.