Add contributing guide

Fixes #391

Author: ileasile

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing Guidelines
+
+There are two main ways to contribute to the project — submitting issues and submitting
+fixes/changes/improvements via pull requests.
+
+## Issue Submission
+
+Please file the issues [here](https://github.com/Kotlin/kotlin-jupyter-libraries/issues).
+
+General types of issues in this project:
+- Descriptor for some library needs to be added
+- The library is obsolete and needs to be removed
+- The library version is out-of-date
+- Descriptor is broken and needs to be fixed
+
+## Submitting Pull Requests
+
+We welcome Pull Requests.
+You can submit them [here](https://github.com/Kotlin/kotlin-jupyter-libraries/pulls).
+
+Some rules apply to the pull requests:
+- If the pull request fixes an issue, please mention it in the pull request title and in the commit message, i.e.
+`Fix #123`
+- If the pull request adds a new library, please check the following things:
+  - `description` and `link` are present in the descriptor. We generate the list of libraries using these values
+  so ensure they are correct
+  - `properties` property is present in the descriptor. It should include all versions of Maven dependencies that are
+  used in `dependencies` section. It should also include hints for the auto-update of these dependencies. I.e.:
+  ```json
+  {
+    "properties": [
+      { "name": "v", "value": "0.1.2" },
+      { "name": "v-renovate-hint", "value": "update: package=<group>:<artifact>" }
+    ]
+  }
+  ```
+  - at least one dependency is present in the `dependencies` list. Version should be a property reference, i.e.
+  `<group>:<artifact>:$v`
+  - Other rules and approaches could be found in [this guide](https://github.com/Kotlin/kotlin-jupyter/blob/master/docs/libraries.md)
+
+## Contacting maintainers
+
+If you encounter any issues or have questions about the project, there are several ways to contact the maintainers:
+
+* Submit an [issue](#issue-submission) on GitHub to report a bug or request a new feature.
+* Mention the maintainer (@ileasile) on GitHub to draw their attention
+  to a specific problem or ask a question.
+* For general inquiries and discussions, use the `#notebooks` channel on
+  the [KotlinLang Slack](https://kotl.in/slack).