add how to contribute

CONTRIBUTING.md

+# How to contribute
+
+First of all, thanks for contributing!
+
+This document provides some basic guidelines for contributing to this repository. To propose
+improvements or fix a bug, feel free to submit a PR.
+
+## Legal requirements
+
+Before we can accept your contributions you have to sign the [Contributor License Agreement][0]
+
+## Prerequisites
+
+To build the Arduino CLI from sources you need the following tools to be available in your local
+enviroment:
+
+* [Go][1] version 1.12 or later
+* [Taskfile][2] to help you run the most common tasks from the command line
+
+If you want to run integration tests you will also need:
+
+* A serial port with an Arduino device attached
+* A working [Python][3] environment, version 3.5 or later
+
+## Building the source code
+
+From the project folder root, just run:
+
+```shell
+task build
+```
+
+The project uses Go modules so dependencies will be downloaded automatically, you should end with
+an `arduino-cli` executable in the same folder.
+
+## Running the tests
+
+There are several checks and test suites in place to ensure the code works as expected but also it
+is written in a way that's consistent across the whole codebase. Such tests can be run one after
+another by running the command:
+
+```shell
+task test
+```
+
+If you want to only run unit tests and skip other checks, you can run:
+
+```shell
+task test-unit
+```
+
+Similarly, if you're only interested in running integration tests, you can do (be sure to read the
+part dedicated to integration tests if something doesn't work):
+
+```shell
+task test-integration
+```
+
+### Integration tests
+
+Being a command line interface, Arduino CLI is heavily interactive and it has to stay consistent
+in accepting the user input and providing the expected output and proper exit codes. On top of this,
+many Arduino CLI features involve communicating with external devices, most likely through a serial
+port, so unit tests can only put our confidence that the code is working so far.
+
+For these reasons, in addition to regular unit tests the project has a suite of integration tests
+that actually run Arduino CLI in a different process and assess the options are correctly
+understood and the output is what we expect.
+
+To run the full suite of integration tests you need an Arduino device attached to a serial port and
+a working Python environment. Chances are that you already have Python installed in your system, if
+this is not the case you can [download][3] the official distribution or use the package manager
+provided by your Operating System.
+
+Some dependencies need to be installed before running the tests and to avoid polluting your global
+Python enviroment with dependencies that might be only used by the Arduino CLI, you can use a
+[virtual environment][4]. There are many ways to manage virtual environments, for example you can
+use a productivity tool called [hatch][5]. First you need to install it (you might need to `sudo`
+the following command):
+
+```shell
+pip3 install --user hatch
+```
+
+Then you can create a virtual environment to be used while working on Arduino CLI:
+
+```shell
+hatch env arduino-cli
+```
+
+At this point the virtual environment was created and you need to make it active every time you
+open a new terminal session with the following command:
+
+```shell
+hatch shell arduino-cli
+```
+
+From now on, every package installed by Python will be confined to the `arduino-cli` virtual
+environment, so you can proceed installing the dependencies required with:
+
+```shell
+pip install -r test/requirements.txt
+```
+
+If the last step was successfull, you should be able to run the tests with:
+
+```shell
+task test-integration
+```
+
+## Pull Requests
+
+In order to ease code reviews and have your contributions merged faster, here is a list of items
+you can check before submitting a PR:
+
+* Create small PRs that are narrowly focused on addressing a single concern.
+* PR titles indirectly become part of the CHANGELOG so it's crucial to provide a good
+  record of **what** change is being made in the title; **why** it was made will go in the
+  PR description, along with a link to a GitHub issue if it exists.
+* write tests for the code you wrote.
+* open your PR against the `master` branch.
+* Maintain **clean commit history** and use **meaningful commit messages**.
+  PRs with messy commit history are difficult to review and require a lot of work to be merged.
+* Your PR must pass all CI tests before we will merge it. If you're seeing an error and don't think
+  it's your fault, it may not be! The reviewer will help you if there are test failures that seem
+  not related to the change you are making.
+
+## Additional settings
+
+If you need to push a commit that's only shipping documentation changes or example files, thus a
+complete no-op for the test suite, please start the commit message with the string **[skip ci]**
+to skip the build and give that slot to someone else who does need it.
+
+If your PR doesn't need to be included in the changelog, please start the PR title with the string
+**[skip changelog]**
+
+[0]: https://cla-assistant.io/arduino/arduino-cli
+[1]: https://golang.org/doc/install
+[2]: https://taskfile.dev/#/installation
+[3]: https://www.python.org/downloads/
+[4]: https://docs.python.org/3/tutorial/venv.html
+[5]: https://github.com/ofek/hatch