Create CONTRIBUTING.md (#169)

CONTRIBUTING.md

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+# Contributing to esphomeyaml
+
+esphomeyaml is a part of esphomelib and is responsible for reading in YAML configuration files,
+converting them to C++ code. This code is then converted to a platformio project and compiled
+with [esphomelib](https://github.com/OttoWinter/esphomelib), the C++ framework behind the project.
+
+For a detailed guide, please see https://esphomelib.com/esphomeyaml/guides/contributing.html#contributing-to-esphomeyaml
+
+Things to note when contributing:
+
+ - Please test your changes :)
+ - If a new feature is added or an existing user-facing feature is changed, you should also 
+   update the [docs](https://github.com/OttoWinter/esphomedocs). See [contributing to esphomedocs](https://esphomelib.com/esphomeyaml/guides/contributing.html#contributing-to-esphomedocs)
+   for more information.
+ - Please also update the tests in the `tests/` folder. You can do so by just adding a line in one of the YAML files
+   which checks if your new feature compiles correctly.
+ - Sometimes I will let pull requests linger because I'm not 100% sure about them. Please feel free to ping
+   me after some time.