Improve images, add asyn directory.

async/ASYNC.md

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+# nanogui: Simple asynchronous demos
+
+Demo of the use of `nanogui` with `uasyncio`:  
+ * `asnano.py` Runs until the usr button is pressed.
+
+Requires [asyn.py](https://github.com/peterhinch/micropython-async/blob/master/asyn.py).
+
+###### [Main README](../README.md)