Added basic CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution Guide
+
+## Required Tools and Dependencies
+
+The backend of FreeDATA is built with python, with a couple dependencies written
+in C/C++, and the frontend is build with VueJS, so you will need Python (from
+3.10 to 3.13) to work on the backend, and node+npm to work on the frontend.
+
+System dependencies required:
+  - python3 (with virtual environment support)
+  - portaudio
+  - hamlib (optional, there is a vendored version bundled with the freedata
+    server, or you can build it from source)
+  - nodejs + npm
+
+Example for debian trixie: `sudo apt update && sudo apt install python3
+python3-venv nodejs npm portaudio19-dev`
+
+Example for MacOS: `brew install python pyenv-virtualenv npm node@24 portaudio`
+
+To fetch the python dependencies create a virtual environment and next use pip:
+
+- Create a virtual environment: `python -m venv venv`
+- Load the virtual environment `source venv/bin/activate`
+- There are several sets of dependencies you can install:
+  - `pip install .` installs only the libraries required to run freedata
+  - `pip install .[test]` installs the libraries required for running and testing
+    freedata
+  - `pip install .[dev]` installs the libraries required for running, testing
+    and linting/checking freedata
+  - `pip install .[build]` installs only the libraries required to run and build
+    the freedata pip module
+  - `pip install .[nuitka]` installs only the libraries required to run and
+    build the freedata windows executable
+
+To fetch the `npm` dependencies go inside `./freedata_gui` and run `npm
+install`. See `./freedata_gui/README.md` for additional informations.
+
+## Linting and Formatting (python-only)
+The python sources must be formatted and checked using `ruff`. There is a CI
+workflow that block all PRs that do not pass the linter/formatter checks.
+
+- To check formatting run `ruff format --check` (lists file that needs formatting)
+- To see what the automatic formatter will change run `ruff format --diff` (shows
+  diff-style changes).
+- To automatically format files run `ruff format`
+
+- To run the checker/linter execute `ruff check`
+- Some errors can be automatically fixed running `ruff check --fix`, but always
+  check and test the changes made by ruff. There are errors that ruff cannot safely
+  fix automatically, you must fix them manually.
+
+## Testing backend
+To run the backend tests run inside the virtual environment `python -m unittest
+discover tests`