Update contributing guidelines for Git practices

Added guidelines for Git history and commit standards.

Author: ManuelSchneid3r

src/gettingstarted/contributing.md

@@ -53,6 +53,8 @@ For a plugin to be accepted it has to satisfy the following **requirements**:
 - The project has an **active maintainer**. 
 - The repository **content is essential** to the functionality of the plugin. Keep in mind that in future somebody else may want to maintain the plugin and for distribution the size of the repository should be minimal. 
 - Icons are **vector graphics** or **128x128 raster images**.
+- Git history has to be linear
+- Git commits have to be [Conventional Commit Spec](https://www.conventionalcommits.org/) compliant (automated versioning and changelogs)
 
 Having a _concise_ `README.md` summarizing features, requirements, limitations, supported platforms and implementation details is recommended.