Add readme

README.txt

+Apache Mynewt project for testing BLE apps using a Seeed Xiao nRF52840.
+
+Build:
+
+> newt upgrade [--shallow 1000, or something]
+> newt build ble-demo
+> newt debug ble-demo
+
+The newt tool comes from https://mynewt.apache.org/download/
+
+License:
+
+Some of the Apache bits are Apache-2.0, the rest is whatever.