chore: Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Sylvia
+
+Sylvia welcomes contributions from everyone in the form of suggestions, bug
+reports, pull requests, and feedback. This document gives some guidance if you
+are thinking of helping us.
+
+## Submitting bug reports and feature requests
+
+When reporting a bug or asking for help, please include enough details so that
+the people helping you can reproduce the behavior you are seeing. For some tips
+on how to approach this, read about how to produce a [Minimal, Complete, and
+Verifiable example](https://stackoverflow.com/help/mcve).
+
+When making a feature request, please clarify what problem you intend to
+solve with the feature, any ideas for how Sylvia could support solving that
+problem, any possible alternatives, and any disadvantages.
+
+## Running checks
+
+Before submitting a pull request, please make sure that the following checks pass:
+
+```sh copy
+cargo test && cargo fmt --check && cargo clippy --all-features -- -D warnings
+```
+
+If anything does not pass, typically, it will be
+easier to iterate and fix it locally than waiting for the CI servers to run
+tests for you.
+
+You also have to run the tests in the `example` directory if your pull request affected the `sylvia-derive` crate:
+
+```sh copy
+cd examples && cargo test
+```
+
+## Commit messages
+
+Sylvia requires the commit messages to follow the
+[Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+specification. This convention makes it easier to understand the changes in the
+project and to automate the release process.
+
+Ideally, the commit message should also reference the issue number it aims to fix.
+
+Supported headers are:
+
+- feat
+- fix
+- revert
+- docs
+- test
+- refactor
+- chore
+- build
+
+```
+feat: Support new type of message (#123)
+
+Additional valuable information about added functionality in the commit.
+```
+
+## Releasing
+
+Sylvia uses the [release-plz](https://github.com/release-plz/release-plz) GitHub action to automate the release process. Instead of manually running `cargo publish`, merge the automatic PR generated by this action, which will take care of the rest.
+
+## Versioning
+
+Sylvia follows the [Semantic Versioning](https://semver.org/) scheme.