Add CONTRIBUTING.md guide (#356)

CONTRIBUTING.md

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+# Juniper Contribution Guide
+
+Juniper is always looking for new contributors, so don't be afraid to jump in and help!  
+The maintainers are happy to provide guidance if required.
+
+To get started, you can look for [issues with the "help wanted" label](https://github.com/graphql-rust/juniper/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22).
+
+## PR Checklist
+
+Before submitting a PR, you should follow these steps to prevent redundant churn or CI failures:
+
+[ ] Ensure proper formatting
+[ ] Run all tests
+[ ] Update the CHANGELOG
+
+### Ensure proper formatting
+
+Consistent formatting is enforced on the CI.
+
+Before you submit your PR, you should run `cargo fmt` in the root directory.
+
+Formatting should be run on the **stable** compiler. 
+(You can do `rustup run stable cargo fmt` when developing on nightly)
+
+### Run all tests
+
+To run all available tests, including verifying the code examples in the book,
+you can use [cargo-make](https://github.com/sagiegurari/cargo-make).
+
+1. Install cargo-make with `cargo install cargo-make`
+2. Run `cargo make ci-flow` in the root directory
+
+### Update the CHANGELOG
+
+Add your changes to the relevant changelog if they affect users in any way.
+Each sub-crate has it's own CHANGELOG.md.
+
+Your changes should be added to a `[master]` section on top of the file.