Added a readme for the gateway plugin since there are 2 versions.

One for Python 2.7 and one for Python 3.7

plugins/telegraf_gateways.md

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+
+Depending on how recent your pfSense install is, you may need to use the python 3.7 version of the plugin.
+If you are running 2.4.5, you probably want telegraf_gateways-3.7.py. They both output he same data in the same format.
+
+All I did was copy telegraf_gateways-3.7.py to /usr/local/bin and rename it to telegraf_gateways.py 
+
+### Python 2.7
+Does /usr/local/bin/python2.7 exist on your pfSense system?  If so, use this
+telegraf_gateways-2.7.py
+
+### Python 3.7
+Does /usr/local/bin/python3.7 exist on your pfSense system?  If so, use this
+telegraf_gateways-3.7.py
+
+If you have both 2.7 and 3.7 on your system, use 3.7