add a very simple dev backend readme

bazarr/README.md

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+
+NOTE: must use python 3.12
+
+If your on mac you may need to change the `python` below to `python3.12`
+
+# Create a virtual environment
+
+python -m venv venv
+
+# Activate the virtual environment
+
+source venv/bin/activate
+
+# Install the dependencies
+
+pip install -r requirements.txt
+
+# Run
+
+python bazarr/main.py --no-update