Add contributing guidelines.

CONTRIBUTING.md

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+## How to contribute to ExDgraph
+
+:+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+The following is a set of guidelines for contributing to ExDgraph. These are mostly guidelines, not rules. Use your best judgment, and feel free to propose changes to this document in a pull request.
+
+#### **Did you find a bug?**
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/ospaarmann/exdgraph/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/ospaarmann/exdgraph/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+#### **Did you write a patch that fixes a bug?**
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+
+#### **Did you fix whitespace, format code, or make a purely cosmetic patch?**
+
+Changes that are cosmetic in nature and do not add anything substantial to the stability, functionality, or testability of ExDgraph will generally not be accepted.
+
+#### **Do you intend to add a new feature or change an existing one?**
+
+* Suggest your change in the #elixir channel in [Dgraph Slack](https://slack.dgraph.io/) and start writing code.
+
+* Do not open an issue on GitHub until you have collected positive feedback about the change. GitHub issues are primarily intended for bug reports and fixes.
+
+#### **Do you have questions about the source code?**
+
+* Ask any question about how to use ExDgraph in the #elixir channel in [Dgraph Slack](https://slack.dgraph.io/).
+
+Thanks! :heart: :heart: :heart:
+
+Ole Spaarmann