Add contribution guide

Author: oliemansm

CONTRIBUTING.md

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+# How to contribute
+
+We're really glad you're reading this, because we need more volunteer developers
+to help with this project! If you haven't already, connect with us on 
+[Spectrum](https://spectrum.chat/graphql-java-kick?tab=posts).
+
+We can use all the help we can get on all our [GraphQL Java Kickstart](https://github.com/graphql-java-kickstart)
+projects. This work ranges from adding new features, fixing bugs, and answering questions to writing documentation. 
+
+## Answering questions and writing documentation
+
+A lot of the questions asked on Spectrum or Github are caused by a lack of documentation.
+We should strive from now on to answer questions by adding content to 
+our [documentation](https://github.com/graphql-java-kickstart/documentation) and referring
+them to the newly created content.
+
+Continuous integration will make sure that the changes are automatically deployed to 
+https://www.graphql-java-kickstart.com.
+
+## Submitting changes
+
+Please send a Pull Request with a clear list of what you've done using the
+[Github flow](https://guides.github.com/introduction/flow/). We can always use more
+test coverage, so we'd love to see that in the pull requests too. And make sure to
+follow our coding conventions (below) and make sure all your commits are atomic 
+(one feature per commit).
+
+## Coding conventions
+
+We use Google Style guides for our projects. See the 
+[Java Style Guide](https://google.github.io/styleguide/javaguide.html) for a detailed
+description. You can download the 
+[IntelliJ Java Google Style](https://github.com/google/styleguide/blob/gh-pages/intellij-java-google-style.xml)
+to import in these settings in IntelliJ.