Add CONTRIBUTING.md

Author: jajik

CONTRIBUTING.md

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+# Contributing to mod\_proxy\_cluster
+
+Before contributing please read our [Code of Conduct](https://github.com/modcluster/mod_proxy_cluster/blob/main/CODE_OF_CONDUCT.md)
+and [README](https://github.com/modcluster/mod_proxy_cluster/blob/main/README.md).
+
+## How to contribute
+
+### Reporting bugs
+
+Please create an issue here in the repository with a clear description and steps to reproduce (if any).
+
+### Providing patches
+
+By contributing a code to the repository, you agree with the
+[Code of Conduct](https://github.com/modcluster/mod_proxy_cluster/blob/main/CODE_OF_CONDUCT.md) and
+the [license](https://github.com/modcluster/mod_proxy_cluster/blob/main/LICENSE).
+
+To contribute, fork this repository and prepare your contribution. Make sure the code adheres to our code style,
+commit messages are clear, and that the workflow pipelines are not failing. Also, make sure that your fork is up to date
+and is not missing any upstream commits. Once you checked all of these, open a pull request against the right branch (see
+the section below) and wait for a review.
+
+#### Branches
+
+* `main` – the default branch for the development, the default target for pull requests
+
+* `1.3.x` – the source for 1.3.x releases, changes from `main` are backported here – open pull requests against this branch
+**only** in cases the changes are applicable to 1.3.x releases only
+