doc: add CONTRIBUTING.md guidelines

CONTRIBUTING.md

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+# How to contribute
+
+## Setup a development environment
+
+1) First of all make sure to have a 3.x python version installed
+2) Clone the source code:
+    `[email protected]:MarketSquare/robotframework-archivelibrary.git`
+3) I suggest to create a python virtual environment:
+```sh
+    cd robotframework-requests/
+    python -m venv venv
+    source venv/bin/activate
+```
+4) Install the library in editing mode and all the test dependencies:
+    `python -m pip install -e`
+5) Run acceptance tests with robot:
+    `robot ./atests`
+6) Run unit tests wiht pytest:
+    `pytest ./utests`
+
+If everything went well now you're ready to go!
+
+## Coding guidelines
+
+Many checks and tests are automatically performed in Continuous Integration with the
+GitHub Actions.
+
+Have a look at the file `.github/workflows/python-app.yml` to see the commands used. 
+
+#### Linting
+
+The project uses flake8 for linting the source code.
+
+#### Unit tests
+
+PyTest is integrated for unit tests that located in `utests/` folder.
+
+#### Acceptance tests
+
+Obviously for acceptance tests Robot Framework is used, files are located in `atests/`.
+
+#### Test Coverage
+
+Test coverage is evaluated for unit and acceptance tests, after test execution 
+`coverage report` command shows you the statistics. 
+
+#### Documentation
+
+Keywords documentation (on Linux) can be updated running the following script:
+
+`doc/generate_doc.sh`