Detailed contribution guidelines. (#192)

* Detailed contribution guidelines.

Commit message style, code formatting and further pull request related information is now more detailed described.

* Credits for `Commit mesages` to spring-data-build.

CONTRIBUTING.md

@@ -1 +1,43 @@
-Please run `./mvnw formatter:format` before sending a pull request.
+# YAVI contribution guidelines
+
+Great to hear you would like to contribute to YAVI! 🥳👍🏽
+
+## How to contribute
+
+Before you contribute your changes please make sure you follow the guidelines in this file.
+
+* Please run `./mvnw formatter:format` to make sure your follow the code style guidelines.
+* Make sure you follow the [Commit messages](#commit-messages) guidelines.
+* Make sure you describe your changes inside the pull request, so we get a better overview why this change is needed.
+* Add few test cases (unit or integration tests) that back your changes.
+
+## Source Code style
+
+In this section we will provide you with information on how to format your code and import the styles of this project.
+
+* You can find IDE settings to import at `src/etc/`.
+* This project uses a maven formatter plugin that will autoformat your code. You can expect that
+  calling `./mvnw formatter:format` is the correct code style. Therefore, please run it before commiting.
+
+## Commit messages
+The following section is copied from [spring-projects/spring-data-build](https://github.com/spring-projects/spring-data-build/blob/main/CONTRIBUTING.adoc) (commit: [245f448](https://github.com/spring-projects/spring-data-build/commit/245f4489bd908a62e5b456c092307ab542e47c16))
+
+The commit message should follow the following style:
+
+* First line contains the summary starting with a capital letter.
+* Finish summary with a dot.
+* In the description, don’t use single line breaks. No manual wrapping. Separate paragraphs by a blank line.
+* During merges, add a reference to the original pull request.
+* To close a GitHub issue, use GitHub syntax to reference issues.
+* Add related references at the very bottom (also see the section on pull requests below).
+
+
+```
+Summary of the commit (try to stay under 80 characters).
+
+Additional explanations if necessary.
+
+Original pull request #?? (can be omitted when there's no ticket using Closes #?? syntax)
+Closes #?? (optionally close tickets)
+Related tickets #??? (optionally refer to related tickets)
+```