create contribution guidelines

CONTRIBUTING.md

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+# How to contribute to sequence-layout
+
+## **Did you find a bug?**
+
+* Ensure the bug was not already reported by searching on GitHub under [Issues](https://github.com/transferwise/sequence-layout/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/transferwise/sequence-layout/issues/new). Be sure to include a title and clear description, as much relevant information as possible, and a code sample 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.
+
+## **Do you intend to add a new feature or change an existing one?**
+
+* Suggest your change in the [Issues](https://github.com/transferwise/sequence-layout/issues) section and add the `feature-request` label.
+
+Thank you for feedback and contributions!
+
+The TransferWise Android team
+
+*These contribution guidelines have been adapted from https://github.com/rails/rails/blob/master/CONTRIBUTING.md*