Add contributing doc (#117)

CONTRIBUTING.md

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+# How to contribute to the Exercism Arturo track
+
+#### **Do you want to add an exercise?**
+
+- **Ensure that someone else isn't already adding it** by searching the [Arturo category on the Exercism forum](https://forum.exercism.org/c/programming/arturo) and the repository's [issues](https://github.com/exercism/arturo/issues) and [pull requests](https://github.com/exercism/arturo/pulls).
+
+- Claim the exercise by adding a comment to [this issue](https://github.com/exercism/arturo/issues/22).
+
+- Follow the [Add a Practice Exercise docs](https://exercism.org/docs/building/tracks/practice-exercises/add).
+
+#### **Do you want to report a bug?**
+
+- **Ensure the bug was not already reported** by searching the [Arturo category on the Exercism forum](https://forum.exercism.org/c/programming/arturo).
+
+- If you're unable to find an open conversation addressing the problem, [open a new one](https://forum.exercism.org/new-topic?category=arturo). Be sure to include a **title and clear description**, as much relevant information as possible, and (when possible) a **code sample**.
+
+#### **Do you want to fix a bug?**
+
+- **Ensure that the bug is [reported](#do-you-want-to-report-a-bug)**.
+  Only start fixing the bug when there is agreement on whether (and how) it should be fixed.
+
+- Fix the bug and [submit a Pull Request](https://exercism.org/docs/building/github/contributors-pull-request-guide) to this repository.
+
+- Ensure the PR description clearly describes the problem and solution.
+  Include a link to the bug's corresponding forum conversation.
+
+- Before submitting, please read the [Contributors Pull Request Guide](https://exercism.org/docs/building/github/contributors-pull-request-guide) and [Pull Request Guide](https://exercism.org/docs/community/being-a-good-community-member/pull-requests).
+
+#### **Do you intend to add a new feature or change an existing one?**
+
+- **Ensure that the feature or change is discussed in the [Arturo category on the Exercism Forum](https://forum.exercism.org/c/programming/arturo).**
+  Only start adding the feature or change when there is agreement on whether (and how) it should be added or changed.
+
+- Add the feature or change and [submit a Pull Request](https://exercism.org/docs/building/github/contributors-pull-request-guide) to this repository.
+
+- Ensure the PR description clearly describes the problem and solution.
+  Include a link to the bug's corresponding forum conversation.
+
+- Before submitting, please read the [Contributors Pull Request Guide](https://exercism.org/docs/building/github/contributors-pull-request-guide) and [Pull Request Guide](https://exercism.org/docs/community/being-a-good-community-member/pull-requests).