Add doc note about threading and use OSThread to make GPIO watching work

Thanks to @mc-hamster for the idea

docs/software/TODO.md

@@ -10,7 +10,7 @@ For app cleanup:
 * DONE write devapi user guide
 * DONE update android code: https://developer.android.com/topic/libraries/view-binding/migration
 * only do wantReplies once per packet type, if we change network settings force it again
-* make gpio watch work, use thread and setup
+* test GPIO watch
 * DONE make hello world example service
 * make python ping command
 * DONE have python tool check max packet size before sending to device

docs/software/plugin-api.md

@@ -49,6 +49,11 @@ The easiest way to get started is:
 * Rebuild with your new messaging goodness and install on the device
 * Use the [meshtastic commandline tool](https://github.com/meshtastic/Meshtastic-python) to send a packet to your board "meshtastic --dest 1234 --ping"
 
+## Threading
+
+It is very common that you would like your plugin to be invoked periodically.
+We use a crude/basic cooperative threading system to allow this on any of our supported platforms.  Simply inherit from OSThread and implement runOnce().  See the OSThread [documentation](/src/concurrency/OSThread.h) for more details.  For an example consumer of this API see RemoteHardwarePlugin::runOnce.
+
 ## Picking a port number
 
 For any new 'apps' that run on the device or via sister apps on phones/PCs they should pick and use a unique 'portnum' for their application.