Add CONTRIBUTING.

Author: fkdosilovic

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# How to contribute
+
+## Preparing the development setup
+
+1. Install Python 3.9
+
+```bash
+sudo add-apt-repository ppa:deadsnakes/ppa
+sudo apt update
+sudo apt install python3.9 python3.9-venv
+```
+
+2. Clone the repo, create and activate a new virtual environment
+
+```bash
+cd judge0-python
+python3.9 -m venv venv
+source venv/bin/activate
+```
+
+3. Install the library and development dependencies
+
+```bash
+pip install -e .[test]
+```