Updated docs adding Particle Mesh Board configurations#6

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aroller wants to merge 2 commits intopkourany:masterfrom aroller:master

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Commits on Mar 10, 2019

added docs to identify the pins for the new particle mesh boards (Xenon,Argon,Boron).
aroller
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Successfully tested with a Particle Argon with the pins identified in the updated Readme. Xenon appears the same so assuming Boron is too. Updated SS_PIN assignment since the docs explain that is t…
aroller
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