Add CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Contributing to Refined Storage
+
+## Versioning
+
+This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## Pull requests
+
+- Keep your PR as small as possible, this makes reviewing easier.
+- Commits serve a clear purpose and have a fitting commit message.
+- Branches are kept up to date by rebasing, preferably.
+- PRs are merged by rebasing the commits on top of the target branch.
+- Changes are added in `CHANGELOG.md`. Please refrain from using technical terminology, keep it user-friendly. The
+  format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## Gitflow
+
+This project uses [Gitflow](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow).
+
+## Releasing
+
+1) Make sure the version number in `build.gradle` is correct.
+2) Merge `develop` to `main`.
+3) Push a tag with the version number (prefixed with `v`).
+
+After releasing:
+
+1) Rename the "Unreleased" section to the correct version number in `CHANGELOG.md`.
+2) Upgrade the version number in `build.gradle`.
+3) Create a new "Unreleased" section in `CHANGELOG.md`.
+
+## Pipelines
+
+### Build
+
+The build pipeline triggers when a commit is pushed to a branch or pull request.
+
+All tests are run and an aggregated code coverage report is created. After that, a SonarQube analysis is run.
+
+### Release
+
+The release pipeline triggers when a tag is pushed. This will run all the steps that our build pipeline does.
+
+After that succeeds, it will publish to GitHub packages.
+
+The "Unreleased" section in `CHANGELOG.md` is parsed and a GitHub release is created with the changelog body and
+relevant artifacts.
+
+After that, a Discord and Twitter notification is sent.