Write short contributing instructions

Author: Oxid15

CONTRIBUTING.md

@@ -2,3 +2,65 @@
 Pull requests and issues are welcome! For major changes, please open an issue first to discuss what you would like to change.
 
 Please make sure to update tests and docs as appropriate.
+
+## Developer setup
+
+### Repo
+
+```bash
+git clone https://github.com/Oxid15/cascade.git
+```
+
+### Installation
+
+Use Python one version behind the latest as recommended.
+Remember to create a venv!
+
+``` bash
+python -m venv ./venv
+source ./venv/bin/activate
+```
+
+```bash
+cd cascade
+pip install -e .
+```
+
+### Testing
+
+Install requirements
+
+```bash
+pip install -r cascade/tests/requirements.txt
+```
+
+Run all tests
+
+```bash
+cd cascade/tests
+pytest
+```
+
+### Documentation
+
+Install requirements
+
+```bash
+pip install -r cascade/docs/requirements.txt
+```
+
+Build the docs
+
+```bash
+cd cascade/docs
+sphinx-build source build
+```
+
+Format of docstrings is `numpy`.
+
+### Code
+
+Cascade uses `flake8`, `black` and `isort`. Be sure that `flake8` passes
+before pushing the code.
+
+Each file should have license notice in the header - be sure it is included.