Adding a contributing guide (#826)

* Create CONTRIBUTING.md

* Add links on how to create issues and PRs

* Incorporate comments

* fix link

Author: JanWeldert

docs/CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# How to contribute to PISA
+
+Thank you for reading this and for considering to contribute to PISA! We always need volunteer developers to help move this project forward.
+
+## Submitting changes
+Found a bug or missing a feature? Don't be afraid to tackle this! If you don't do it, maybe no one will.
+
+#### Create an issue
+In any case, it is a good first step to document what you have found (or what is missing) in a GitHub issue. [Here](https://docs.github.com/en/issues/tracking-your-work-with-issues/creating-an-issue) is how to do it.
+
+#### Close an issue
+You are more than welcome to work on something already documented! But remember, the best way to close an issue is with a pull request.
+
+#### Open a pull request
+Once you implemented your changes, you can start a PR. You can start this early, even when you are still working on your code, to get feedback and let people know what you are working on. Use your own fork and a dedicated branch for your PR. [Here](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request) is how to do it.
+
+#### Review a pull request
+Everyone can (and should) review PRs opened by other contributors. This input is very valuable for the PR creator as well as other reviewers.
+
+## Coding conventions
+Coding conventions can be found in [Conventions](https://github.com/icecube/pisa/blob/master/pisa/general_conventions.md).
+
+## Testing
+If you add a feature, add a test that proves it works. More about tests can be found in [Conventions](https://github.com/icecube/pisa/blob/master/pisa/general_conventions.md).
+
+## Ask for help
+If you are stuck with your code or have some general questions, feel free to ask other people on slack (e.g. in the pisa-help channel). If you are not a member of the IceCube collaboration, you can create an issue with your question.
+
+**Thanks a lot and good coding :)**