Create External Notification documentation.

docs/software/plugins/ExternalNotification.md

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+Documentation for ExternalNotification Plugin
+
+# Configuration
+
+These are the settings that can be configured.
+
+    ext_notification_plugin_enabled
+        Is the plugin enabled?
+        
+        0 = Disabled (Default)
+        1 = Enabled
+
+    ext_notification_plugin_active
+        Is your external circuit triggered when our GPIO is low or high?
+
+        0 = Active Low (Default)
+        1 = Active High
+
+    ext_notification_plugin_alert_message
+        Do you want to be notified on an incoming message?
+
+        0 = Disabled (Default)
+        1 = Alert when a text message comes
+
+    ext_notification_plugin_alert_bell
+        Do you want to be notified on an incoming bell?
+
+        0 = Disabled (Default)
+        1 = Alert when the bell character is received
+
+    ext_notification_plugin_output
+        What GPIO is your external circuit attached?
+
+        GPIO of the output. (Default = 13)
+
+    ext_notification_plugin_output_ms
+        How long do you want us to trigger your external circuit?
+    
+        Amount of time in ms for the alert. Default is 1000.
+
+
+# Usage Notes
+
+For basic usage, start with:
+
+	ext_notification_plugin_enabled = 1
+    ext_notification_plugin_alert_message = 1
+    
+Depending on how your external configured is configured, you may need to set the active state to true.
+
+	ext_notification_plugin_active = 1
+    
+# External Hardware
+
+Be mindful of the max current sink and source of the esp32 GPIO. The easiest devices to interface with would be either an LED or Active Buzzer.
+
+    
+# Known Problems
+
+* This won't directly support an passive (normal) speaker as it does not generate any audio wave forms.
+* This currently only supports the esp32. Other targets may be possible, I just don't have to test with.
+
+# Need more help?
+
+Go to the Meshtastic Discourse Group if you have any questions or to share how you have used this.
+
+https://meshtastic.discourse.group