Create SerialPlugin.md

docs/software/plugins/SerialPlugin.md

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+# SerialPlugin
+
+A simple interface to send messages over the mesh network by sending strings
+over a serial port.
+
+Default is to use RX GPIO 16 and TX GPIO 17.
+
+
+# Basic Usage:
+
+        1) Enable the plugin by setting serialplugin_enabled to 1.
+        2) Set the pins (serialplugin_rxd / serialplugin_rxd) for your preferred RX and TX GPIO pins.
+           On tbeam, recommend to use:
+                RXD 35
+                TXD 15
+        3) Set serialplugin_timeout to the amount of time to wait before we consider
+           your packet as "done".
+        4) (Optional) In SerialPlugin.h set the port to PortNum_TEXT_MESSAGE_APP if you want to
+           send messages to/from the general text message channel.
+        5) Connect to your device over the serial interface at 38400 8N1.
+        6) Send a packet up to 240 bytes in length. This will get relayed over the mesh network.
+        7) (Optional) Set serialplugin_echo to 1 and any message you send out will be echoed back
+           to your device.
+
+# TODO (in this order):
+
+* Define a verbose RX mode to report on mesh and packet infomration.
+        - This won't happen any time soon.
+
+# Known Problems
+
+    * Until the plugin is initilized by the startup sequence, the TX pin is in a floating
+      state. Device connected to that pin may see this as "noise".
+    * Will not work on NRF and the Linux device targets.
+    
+# Need help?
+
+Need help with this plugin? Post your question on the Meshtastic Discourse:
+
+https://meshtastic.discourse.group