Create CONTRIBUTING.md (#109)

CONTRIBUTING.md

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+# Contributing to Dokkatoo
+
+Thank you for your interest in contributing to
+[Dokkatoo](https://github.com/adamko-dev/dokkatoo/)!
+We appreciate your help in making this project better.
+
+## Local Development Setup
+
+To get started with local development, follow these steps:
+
+1. Clone the repository:
+    ```shell
+    git clone https://github.com/adamko-dev/dokkatoo.git
+    ```
+2. Navigate to the project directory:
+    ```shell
+    cd dokkatoo
+3. Build the project:
+    ```shell
+    ./gradlew build
+    ```
+
+Here are some additional helpful commands:
+
+* Run tests:
+    ```shell
+    ./gradlew check
+    ```
+* Publish the project to a project-local directory, to verify publication works:
+    ```shell
+    ./gradlew publishToTestMavenRepo
+    ```
+* Update the
+  [Binary Compatibility Validator](https://github.com/Kotlin/binary-compatibility-validator)
+  API dump:
+    ```shell
+    ./gradlew apiDump
+    ```
+
+## Adding New Features
+
+When adding new features, it's generally recommended to
+[create an issue](https://github.com/adamko-dev/dokkatoo/issues/new)
+first before starting any work.
+This allows for better discussion and alignment with the project's goals. Creating an issue
+can help clarify the purpose and prevent duplicate work.
+
+In some cases, making a PR first can be helpful to demonstrate the desired result and
+initiate a discussions. Try to keep the PR small and focused.
+
+If you decide to work on an existing issue, please comment on the issue to let others know that you
+are taking it up. This will avoid unnecessary duplication of effort.
+
+## Coding Guidelines
+
+We have the following guidelines for writing code:
+
+* The code style follows the official
+  [Kotlin Coding conventions](https://kotlinlang.org/docs/coding-conventions.html#source-file-names),
+  with a few differences.
+
+  The most significant difference is that the indent is 2 spaces, not 4.
+
+  The code style is committed in IntelliJ format the repo, in the
+  directory [.idea/codeStyles](./.idea/codeStyles).
+* Follow
+  the [Library creators' guidelines](https://kotlinlang.org/docs/jvm-api-guidelines-introduction.html)
+* ABI changes are tracked using
+  [Binary Compatibility Validator](https://github.com/Kotlin/binary-compatibility-validator).
+* Ideally all changes should have automated test coverage.
+
+## Commit Guidelines
+
+Each commit should have a clear and descriptive commit message.
+
+## Pull Requests
+
+When submitting a pull request, please follow these guidelines:
+
+* Each PR should have a clear title and description (bullet points are preferred)
+  explaining the purpose of the changes.
+* Include the issue number(s) related to the PR, if applicable.
+
+PRs will be squashed into a single commit before merging.
+This will allow us to maintain a clean commit history.
+
+Thank you for your contribution!