Add contribution guideline

[brainelectronics]

Mention steps to be execucted locally before pushing changes in a PR. Maybe add also precommit hooks at least for flake8

          @beyonlo I've published https://test.pypi.org/project/micropython-modbus/2.4.0rc62.dev56/ for testing. 

@GimmickNG please check flake8 before pushing changes, builds are not even reaching the tests anymore

pip install -r requirements-test.txt
flake8 .
yamllint .

# tests can be locally checked by
docker build --tag micropython-test --file Dockerfile.tests .
docker compose up --build --exit-code-from micropython-host
docker compose -f docker-compose-tcp-test.yaml up --build --exit-code-from micropython-host
docker compose -f docker-compose-rtu.yaml up --build --exit-code-from micropython-host
docker compose -f docker-compose-rtu-test.yaml up --build --exit-code-from micropython-host

Originally posted by @brainelectronics in https://github.com/brainelectronics/micropython-modbus/issues/56#issuecomment-1488091972