Update README.md

Author: Alasdair Allan

README.md

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 # MicroPython Examples
 
 Examples to accompany the "Pico Python SDK" book.
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+## Contributions
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+While we welcome pull requests to contribute further example code, please do not ink to personal sites or to your social media. Contributions which are fully documented with an AsciiDoc description and a Fritzing wiring diagram, e.g. see the [NeoPixel Ring](https://github.com/raspberrypi/pico-micropython-examples/tree/master/pio/neopixel_ring) example, stand more chance of inclusion.
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+Our example code is under the BSD-3-Clause License. Any contributions must be under the same license.