Add initial contributing guidelines

CONTRIBUTING.md

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+# Contributing
+
+Want to contribute to gqlgen? Here are some guidelines for how we accept help.
+
+## Getting in Touch
+
+Our [gitter](https://gitter.im/gqlgen/Lobby) channel is the best place to ask questions or get advice on using gqlgen.
+
+## Reporting Bugs and Issues
+
+ We use [GitHub Issues](https://github.com/99designs/gqlgen/issues) to track bugs, so please do a search before submitting to ensure your problem isn't already tracked.
+
+### New Issues
+
+Please provide the expected and observed behaviours in your issue. A minimal GraphQL schema or configuration file should be provided where appropriate.
+
+## Proposing a Change
+
+If you intend to implement a feature for gqlgen, or make a non-trivial change to the current implementation, we recommend [first filing an issue](https://github.com/99designs/gqlgen/issues/new) marked with the `proposal` tag, so that the engineering team can provide guidance and feedback on the direction of an implementation.  This also help ensure that other people aren't also working on the same thing.
+
+Bug fixes are welcome and should come with appropriate test coverage.
+
+New features should be made against the `next` branch.
+
+### License
+
+By contributing to gqlgen, you agree that your contributions will be licensed under its MIT license.