#20 Create CONTRIBUTING.md (#21)

* #20 Create CONTRIBUTING.md
- Added new file.

CONTRIBUTING.md

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+# How to contribute to ScAPI
+
+## **Did you find a bug?**
+
+* **Ensure the bug has not already been reported** by searching our **[GitHub Issues](https://github.com/AbsaOSS/ScAPI/issues)**.
+* If you are unable to find an open issue describing the problem, use the **Bug report** template to open a new one. Tag it with the **bug** label.
+
+## **Do you want to request a new feature?**
+
+* **Ensure the feature has not already been requested** by searching our **[GitHub Issues](https://github.com/AbsaOSS/ScAPI/issues)**.
+* If you are unable to find the feature request, create a new one. Tag it with the **request** label.
+
+## **Do you want to implement a new feature or fix a bug?**
+
+* Check _Issues_ logs for the feature/bug. Check if someone isn't already working on it.
+  * If the feature/bug is not yet filed, please write it up first:
+    * **"Life, the universe and everything"**
+* Fork the repository.
+* We follow a simplified [**GitFlow**](https://nvie.com/posts/a-successful-git-branching-model/) branching strategy:
+  * Cut your branch from `master`, add the _GitHub Issue_ in the branch name:
+    * **feature/42-life-universe-everything**
+    * **bugfix/42-life-universe-everything**
+* Code away. Ask away. Work with us.
+  * Commit messages should start with a reference to the GitHub Issue and provide a brief description in the imperative mood:
+    * **"#42 Answer the ultimate question"**
+  * Don't forget to write tests for your work.
+* After finishing everything, push to your forked repo and open a Pull Request to our `master` branch:
+  * Pull Request titles should start with the Github Issue number:
+    * **"42 Life, the universe and everything"**
+  * Ensure the Pull Request description clearly describes the solution.
+  * Connect the PR to the _Issue_
+
+#### Thanks!
+
+The AbsaOSS team