Add contribution guidelines

Author: dominikbraun

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contributing
+
+## Feature requests
+
+Desire a new timetrace feature? Just propose your idea by
+[creating an issue](https://github.com/dominikbraun/timetrace/issues/new).
+
+Good feature proposals ...
+* explain the problem that the feature solves.
+* explain why it would be a desirable feature for the majority of users.
+
+## Reporting issues
+
+If you encounter an unexpected behavior or a bug, feel free to
+[file an issue](https://github.com/dominikbraun/timetrace/issues/new). When you
+do so, please make sure to ...
+* include version information from the output of `timetrace version`.
+* provide steps to reproduce the behavior.
+
+## Code contributions
+
+### Setting up local development
+
+Developing timetrace only requires [Go 1.16](https://golang.org/dl/).
+
+1. Fork the repository.
+2. Clone your forked repository.
+3. Run `go run . version` to verify that everything works.
+
+### Coding conventions
+
+* All code has to follow the [Effective Go](https://golang.org/doc/effective_go.html) guidelines.
+* All code has to be formatted with `gofmt -s`.
+* Exported types and methods should be documented briefly. Explain what they're doing, not what they are.
+
+### Contributing changes
+
+1. Make your changes. If needed, [write tests](core/timetrace_test.go).
+2. Run `go run . <command>` for testing your changes.
+3. Run `go test ./...` to verify that all tests pass.
+4. Commit your changes and open a PR.