Create CONTRIBUTING.md (#3926)

Co-authored-by: toasterparty <toasterparty@derpymail.org>

CONTRIBUTING.md

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+## How to write a good pull request
+
+- Describe your motivation for your proposed changes
+- Describe in detail what you changed
+- Describe how you tested the changes
+- Describe any follow-up work that is related but outstanding and describe any concerns you might have about your proposed changes
+
+## Target branch
+
+Pull requests should target the `main` branch. If the change is relevant for a hotfix release, make a note in the PR description,
+so it can be cherry-picked after merge.
+
+## Adding to CHANGELOG
+
+Changelog entries are to be written within the scope of a specific upcoming release.
+In practice, changelog entries should always go under the unreleased section (the first one) of [CHANGELOG.md](CHANGELOG.md).
+As such, changelog entries and the specific language used in them must be with respect to the previous release.
+
+If a feature changes multiple times in development, the existing entry should be edited rather than appended if it is still unreleased at the time of writing.
+
+### Example of changes that should have changelog:
+
+- Any database change for a stable game
+- Any GUI change (that isn't hidden by `preview mode` or belongs to an experimental game).
+- Changes to multiworld, presets, generator or solver that causes visible changes in the result
+- Patcher changes affecting gameplay
+- Bugfixes and other stability/performance improvements
+
+### Example of changes that should _not_ have a changelog:
+
+- Changes to experimental games
+- Anything only visible when running from source
+- Code refactoring
+- Dependency changes (except for patcher dependencies, since these can change the results!)
+
+## How to write a good Changelog Entry
+
+- Use the full name of areas
+- Mention all new tricks/difficulty pair and in which areas.
+- It should be discernable which areas are affected when reading the Changelog alone.
+It is acceptable if users need to open the Data Visualizer to understand the exact details of what changed in those areas, however.
+
+## Need some help?
+
+Join the [Randovania Discord](https://discord.gg/M23gCxj6fw), grab the `@Dev Chat` role in `#roles` and chat with us in the `dev-talk` channels.
+
+We're always happy to help new people with their changes, be it pointing where to change things, brainstorming how something should be, or anything else!