docs: Add contribution guide

docs/CONTRIBUTING.md

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+# How to contribute
+
+We're delighted that you're considering contributing to Exposed!
+
+There are multiple ways you can contribute:
+
+* Code
+* Documentation
+* Community Support
+* Issues and Feature Requests
+
+This project and the corresponding community is governed by
+the [JetBrains Open Source and Community Code of Conduct](https://confluence.jetbrains.com/display/ALL/JetBrains+Open+Source+and+Community+Code+of+Conduct).
+Independently of how you'd like to contribute, please make sure you read and comply with it.
+
+### Code
+
+#### Pull Requests
+
+Contributions are made using Github [pull requests](https://help.github.com/en/articles/about-pull-requests):
+
+1. Fork the Exposed repository, because imitation is the sincerest form of flattery.
+2. Clone your fork to your local machine.
+3. Create a new branch for your changes.
+4. [Create](https://github.com/JetBrains/Exposed/compare) a new PR with a request to merge to the **master** branch.
+5. Ensure that the description is clear and refers to an existing ticket/bug if applicable, prefixing the description with
+   EXPOSED-{NUM}, where {NUM} refers to the YouTrack issue.
+6. When contributing a new feature, provide motivation and use-cases describing why
+   the feature not only provides value to Exposed, but also why it would make sense to be part of the Exposed framework itself.
+7. If the contribution requires updates to documentation (be it updating existing contents or creating new one), please
+   file a new ticket on [YouTrack](https://youtrack.jetbrains.com/issues/EXPOSED).
+8. Make sure any code contributed is covered by tests and no existing tests are broken.
+
+#### Style Guides
+
+A few things to remember:
+
+* Your code should conform to the official [Kotlin code style guide](https://kotlinlang.org/docs/reference/coding-conventions.html)
+  except that star imports should be always enabled.
+  (ensure Preferences | Editor | Code Style | Kotlin, tab **Imports**, both `Use import with '*'` should be checked).
+* Every new source file should have a copyright header.
+* Every public API (including functions, classes, objects and so on) should be documented,
+  every parameter, property, return types and exceptions should be described properly.
+
+#### Commit messages
+
+* Commit messages should be written in English.
+* Their title should be prefixed according to [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/#summary).
+* They should be written in present tense using imperative mood ("Fix" instead of "Fixes", "Improve" instead of "Improved").
+  See [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/).
+* When applicable, prefix the commit message with EXPOSED-{NUM} where {NUM} represents the YouTrack issue number.
+* Add the related bug reference to a commit message (bug number after a hash character between round braces).
+
+### Documentation
+
+There are multiple ways in which you can contribute to Exposed docs:
+
+- Create an issue in [YouTrack](https://youtrack.jetbrains.com/issues/EXPOSED).
+- Submit a pull request with proposed changes.
+
+### Community Support
+
+If you'd like to help others, please join our Exposed [channel](https://kotlinlang.slack.com/archives/C0CG7E0A1) on the Kotlin Slack workspace and
+help out. It's also a great way to learn!
+
+### Issues and Feature Requests
+
+If you encounter a bug or have an idea for a new feature, please submit it to us through [YouTrack](https://youtrack.jetbrains.com/issues/EXPOSED),
+our issue tracker.
+
+Before submitting an issue or feature request, please search YouTrack's existing issues to avoid reporting duplicates.
+
+When submitting an issue or feature request, please provide as much detail as possible, including a clear and concise description of the problem or
+desired functionality, steps to reproduce the issue, and any relevant code snippets or error messages.
+
+Thank you for your cooperation and for helping to improve Exposed.