Add CONTRIBUTING.md

Source file https://github.com/dry-rb/template-gem/blob/HEAD/CONTRIBUTING.md

CONTRIBUTING.md

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+# Issue Guidelines
+
+## Reporting bugs
+
+If you found a bug, report an issue and describe what's the expected behavior versus what actually happens. If the bug causes a crash, attach a full backtrace. If possible, a reproduction script showing the problem is highly appreciated.
+
+## Reporting feature requests
+
+Report a feature request **only after discussing it first on [discourse.dry-rb.org](https://discourse.dry-rb.org)** where it was accepted. Please provide a concise description of the feature.
+
+## Reporting questions, support requests, ideas, concerns etc.
+
+**PLEASE DON'T** - use [discourse.dry-rb.org](https://discourse.dry-rb.org) instead.
+
+# Pull Request Guidelines
+
+A Pull Request will only be accepted if it addresses a specific issue that was reported previously, or fixes typos, mistakes in documentation etc.
+
+Other requirements:
+
+1) Do not open a pull request if you can't provide tests along with it. If you have problems writing tests, ask for help in the related issue.
+2) Follow the style conventions of the surrounding code. In most cases, this is standard ruby style.
+3) Add API documentation if it's a new feature
+4) Update API documentation if it changes an existing feature
+5) Bonus points for sending a PR which updates user documentation in the `docsite` directory
+
+# Asking for help
+
+If these guidelines aren't helpful, and you're stuck, please post a message on [discourse.dry-rb.org](https://discourse.dry-rb.org).