docs: Add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,66 @@
+# Contributing to kotlinx.serialization.msgpack
+
+First of all, thanks for taking the time to contribute to the project!
+
+Following is a basic set of guidelines for contributing to this repository and instructions to make it as easy as possible.
+
+> Parts of this guidelines are taken from https://github.com/atom/atom/blob/master/CONTRIBUTING.md
+
+#### Table of contents
+
+[Code of Conduct](#code-of-conduct)
+[Asking questions](#asking-questions)
+[Styleguides](#styleguides)
+ - [Commit message](#commit-messages)
+ - [Lint](#lint)
+[Additional info](#additional-info)
+
+## Code of Conduct
+
+Code of conduct is [available in the repository](CODE_OF_CONDUCT.md)
+
+## Asking questions
+
+For asking questions, please make sure to use [**Discussions**](https://github.com/esensar/kotlinx-serialization-msgpack/discussions) instead of **Issues**.
+
+## Styleguides
+
+### Commit messages
+ - Use the present tense ("Add feature" not "Added feature")
+ - Use the imperative mood ("Move cursor to..." not "Moves cursor to...")
+ - Limit the first line to 72 characters or less
+ - Reference issues and pull requests liberally after the first line
+ - Project uses [Karma commit message format](http://karma-runner.github.io/6.0/dev/git-commit-msg.html)
+
+### Lint
+
+This project uses [ktlint](https://ktlint.github.io/), with [ktlint-gradle](https://github.com/JLLeitschuh/ktlint-gradle) plugin. This makes it easy to set up ktlint locally (ktlint is run on every pull request, together with full test suite).
+
+To run ktlint locally, run:
+```
+./gradlew ktlintCheck
+```
+
+It is also possible to fix some of the issues automatically, by running:
+```
+./gradlew ktlintFormat
+```
+
+It is recommended to set up git pre-commit hook to format all changed files:
+```
+./gradlew addKtlintFormatGitPreCommitHook
+```
+
+It is also possible to do **only** check, instead of format:
+```
+./gradlew addKtlintCheckGitPreCommitHook
+```
+
+If using IntelliJ IDEA IDE, ktlint rules can also be applied to IntelliJ IDEA project:
+```
+./gradlew ktlintApplyToIdea
+```
+
+## Additional info
+
+Since this project just implements [MsgPack](https://msgpack.org/) serialization format for [kotlinx.serialization](https://github.com/Kotlin/kotlinx.serialization) library, make sure that all changes made stay as close as possible to API of default kotlinx.serialization formats, as well as stick to [MsgPack spec](https://github.com/msgpack/msgpack/blob/master/spec.md).

README.md

@@ -62,3 +62,11 @@ Library should be used in same way as any other kotlinx.serialization library. C
         ) // Outputs: SampleClass(testString=def, testInt=123, testBoolean=true)
     }
 ```
+
+## Contributing
+
+Check out [contributing guidelines](CONTRIBUTING.md).
+
+## License
+
+[MIT](LICENSE)