Added a Contribution Guide

CONTRIBUTING.md

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+# Contributing Guidelines
+
+There are two main ways to contribute to the project — submitting issues and submitting
+fixes/changes/improvements via pull requests.
+
+## Submitting issues
+
+Both bug reports and feature requests are welcome.
+Submit issues [here](https://github.com/Kotlin/dataframe/issues).
+
+* Search for existing issues to avoid reporting duplicates.
+* When submitting a bug report:
+    * Test it against the most recently released version. It might have been already fixed.
+    * Include the code that reproduces the problem or attach the link to the repository with the project which fully reproduces the problem.
+    * However, don't put off reporting any weird or rarely appearing issues just because you cannot consistently
+      reproduce them.
+    * If the bug is in behavior, then explain what behavior you've expected and what you've got.
+* When submitting a feature request:
+    * Explain why you need the feature — what's your use-case, what's your domain.
+    * Explaining the problem you face is more important than suggesting a solution.
+      Report your issue even if you don't have any proposed solution.
+    * If there is an alternative way to do what you need, then show the code of the alternative.
+
+## Submitting PRs
+
+We love PRs. Submit PRs [here](https://github.com/Kotlin/dataframe/pulls).
+However, please keep in mind that maintainers will have to support the resulting code of the project,
+so do familiarize yourself with the following guidelines.
+
+* All development (both new features and bug fixes) is performed in the `master` branch.
+    * Base PRs against the `master` branch.
+    * PR should be linked with the issue,
+      excluding minor documentation changes, the addition of unit tests, and fixing typos.
+* If you make any code changes:
+    * Follow the [Kotlin Coding Conventions](https://kotlinlang.org/docs/reference/coding-conventions.html).
+    * [Build the project](#building) to make sure it all works and passes the tests.
+* If you fix a bug:
+    * Write the test the reproduces the bug.
+    * Fixes without tests are accepted only in exceptional circumstances if it can be shown that writing the
+      corresponding test is too hard or otherwise impractical.
+* If you introduce any new public APIs:
+    * All new APIs must come with documentation and tests.
+    * If you plan API additions, then please start by submitting an issue with the proposed API design
+      to gather community feedback.
+    * [Contact the maintainers](#contacting-maintainers) to coordinate any big piece of work in advance via submitting an issue.
+* If you fix documentation:
+    * If you plan extensive rewrites/additions to the docs, then please [contact the maintainers](#contacting-maintainers)
+      to coordinate the work in advance.
+    * Also, we have a special simple [guide](https://github.com/Kotlin/dataframe/blob/master/docs/contributions.md) how to contribute in the documentation.
+
+## PR workflow
+
+0. Contributor build the library locally and run all unit test via Gradle task `dataframe:test`
+   (see the ["Building"](#building) chapter).
+1. Contributor submits the PR if the local build is successful and tests are green.
+2. Reviewer put his name to the "Reviewers" section of the proposed PR at the start of the review process.
+3. Reviewer leaves the comments or marks the PR with the word "LGTM."
+4. Contributor answers the comments or fixes the proposed PR.
+5. Reviewer marks the PR with the word "LGTM."
+6. Maintainer could suggest merging the master branch to the PR branch a few times due to changes in the `master` branch.
+7. Maintainer runs TC builds (unit tests and examples as integration tests).
+8. The TC writes the result (passed or not passed) to the PR checks at the bottom of the proposed PR.
+9. If it is possible, maintainers share the details of the failed build with the contributor.
+10. Maintainer merges the PR if all checks are successful and there is no conflict with the master branch.
+
+## How to fix an existing issue
+
+* If you are going to work on the existing issue:
+    * Comment on the existing issue if you want to work on it.
+    * Wait till it is assigned to you by [maintainers](#contacting-maintainers).
+    * Ensure that the issue not only describes a problem, but also describes a solution that had received a positive feedback. Propose a solution if there isn't any.
+* If you are going to submit your first PR in this project:
+    * Find tickets with the label ["good first issue"](https://github.com/Kotlin/dataframe/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22+no%3Aassignee)
+      which are not assigned to somebody.
+    * Learn the [`examples`](https://github.com/Kotlin/dataframe/tree/master/examples) module, submit new interesting example or improve documentation for one of them.
+* If you are ready to participate in library design and in new experiments, find tickets with the label
+  ["research"](https://github.com/Kotlin/dataframe/issues?q=is%3Aissue+is%3Aopen+label%3Aresearch)
+  or join to our [discussions](https://github.com/Kotlin/dataframe/discussions).
+
+## Environment requirements
+
+JDK >= 11 referred to by the `JAVA_HOME` environment variable.
+
+## Building
+
+This library is built with Gradle.
+
+* Run `./gradlew build` to build. It also runs all the tests.
+* Run `./gradlew <module>:test` to test the module you are looking at to speed
+  things up during development.
+
+You can import this project into IDEA, but you have to delegate build actions
+to Gradle (in Preferences -> Build, Execution, Deployment -> Build Tools -> Gradle -> Runner)
+
+## Contacting maintainers
+
+* If something cannot be done, not convenient, or does not work &mdash; submit an [issue](#submitting-issues).
+* To attract attention to the problem or raised question or a new comment, mention one of us on the Github: @koperagen @Jolanrensen @zaleslaw @ileasile
+* Discussions and general inquiries &mdash; use `#datascience` channel in [KotlinLang Slack](https://kotl.in/slack).