SafeDetect / Mbed OS tx_serial_from_pi

this works for reading in data from pi via usb

Diff: CONTRIBUTING.md

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+# Contributing to Mbed OS
+
+Mbed OS is an open-source, device software platform for the Internet of Things. Contributions are an important part of the platform, and our goal is to make it as simple as possible to become a contributor.
+
+To encourage productive collaboration, as well as robust, consistent and maintainable code, we have a set of guidelines for [contributing to Mbed OS](https://os.mbed.com/docs/latest/reference/contributing.html).