Add CONTRIBUTING.md

Author: Sebastian Wilgosz

CONTRIBUTING.md

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+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/driggl/jsonapi_errors_handler.
+
+**How to contribute:**
+
+1. Create Issue on Github.
+2. Fork repository
+3. Install [Rubocop](https://github.com/rubocop-hq/rubocop) - make sure you run it before commiting changes
+4. Commit changes
+  - Keep commits small and atomic
+  - Start commit message from keywords (Add/Remove/Change/Refactor/Move/Rename/Upgrade/Downgrade)
+  - Keep commits imperative style
+  - Mention issue number (i.e "Resolves: #3")
+5. Make sure you cover your change with tests.
+6. Create Pull Request
+
+**Coding Guidelines**
+
+- We use [Rubocop](https://github.com/rubocop-hq/rubocop) and Ruby style guides for linting `*.rb` files.
+- We use [Remark Lint](https://github.com/remarkjs/remark-lint) for linting `*.md` files.