Add contribution guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+
+The main way to help `kotlinx-datetime` is to describe use cases that
+the library doesn't cover well or at all.
+
+## Submitting issues
+
+Bug reports, feature requests, usability complaints, quality-of-life
+suggestions are all welcome!
+Submit issues [here](https://github.com/Kotlin/kotlinx-datetime/issues).
+
+Questions about usage and general inquiries are better suited for [StackOverflow](https://stackoverflow.com)
+or the `#kotlinx-datetime` channel in [KotlinLang Slack](https://surveys.jetbrains.com/s3/kotlin-slack-sign-up).
+
+## Submitting PRs
+
+* Pull requests that improve code samples or documentation are very welcome.
+* We reject pull requests that introduce new APIs.
+  Please describe your use case under an existing issue or open a new one
+  instead of going straight to implementing a new API in `kotlinx-datetime`.
+  - Datetime APIs are fraught with peril.
+    What may look like a small convenience function may hide an incorrect
+    behavior that will surface once a year.
+  - The core Kotlin libraries go for a consistent naming scheme that is
+    tricky to coordinate with third-party contributors.
+* Do send us pull requests with bugfixes!
+  Those should be accompanied by a test verifying the correct behavior.