Add how to contribute.

Author: Nathan Seidle

CONTRIBUTING.md

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+### How to Contribute
+
+Thank you so *much* for offering to help out. We truly appreciate it.
+
+If you'd like to contribute, start by searching through the [issues](https://github.com/sparkfun/SparkFun_Ublox_Arduino_Library/issues) and [pull requests](https://github.com/sparkfun/SparkFun_Ublox_Arduino_Library/pulls) to see whether someone else has raised a similar idea or question.
+
+If you decide to add a feature to this library, please create a PR and follow these best practices:
+
+* Change as little as possible. Do not sumbit a PR that changes 100 lines of whitespace. Break up into multiple PRs if necessary.
+* If you've added a new feature document it with a simple example sketch. This serves both as a test of your PR and as a quick way for users to quickly learn how to use your new feature.
+* If you add new functions also add them to keywords.txt so that they are properly highlighted in Arduino. [Read more](https://www.arduino.cc/en/Hacking/libraryTutorial).
+
+## Style guide
+
+Please read and follow the [Arduino API style guide](https://www.arduino.cc/en/Reference/APIStyleGuide). Also read and consider the [Arduino style guide](https://www.arduino.cc/en/Reference/StyleGuide).