Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Every Pull Request is welcome.
+
+## Upgrading the Version
+
+To upgrade the LimeSurvey Version the ARG variable needs to be changed.
+
+```bash
+$ grep Agrep ARG apache/Dockerfile
+ARG version='3.7.0+180418'
+```
+
+Since this is a reoccuring and boring task, a script is provided.
+
+```bash
+# Dependencies
+python3 -m venv .venv
+source .venv/bin/activate
+pip3 install -r requirements.txt
+
+# Upgrades to latest Limesurvey version
+./upgrade.py
+```
+
+## Testing
+
+In order to make sure the image works as promised, some tests are provided:
+
+```bash
+./tests/run.sh
+```
+
+For further information:  https://github.com/GoogleContainerTools/container-structure-test