Add CONTRIBUTING.md

Author: eexit

CONTRIBUTING.md

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+# How to contribute
+
+Thank you for reading this, this project is open and my only wish is to make it better.
+
+Before you get started, be aware that I'm not a Go expert and still in the learning curve. I believe some pieces of this software could be done in a more idiomatic and elegant way, and my desire is to learn from more experienced Go engineers.
+
+## Testing
+
+This project is heavily tested: coverage near 100% and I would like to keep it that way. If some code is not testable, this probably means the design is not right.
+
+How to test the coverage?
+
+```bash
+go test ./... -count=1 -coverprofile=coverage.txt -covermode=atomic
+```
+
+Then, visualize the coverage report:
+
+```bash
+go tool cover -html=coverage.txt
+```
+
+## Linting
+
+This projects uses the tool [golangci-lint](https://golangci-lint.run/). You can [install it locally](https://golangci-lint.run/usage/install/#local-installation) and run it before you commit your code.
+
+You may adapt the configuration to improve the standard but avoid lowering it.