Created first draft of the contributing file

CONTRIBUTING.md

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+# Contributing
+
+This file will guide you through the process of getting to project up and running, in order to provide coding contributions.
+
+You will also see how to ensure the code fulfills the expected code checks, and how to end up creating a pull request.
+
+## System dependencies
+
+The project provides all its dependencies as docker containers through a docker-compose configuration.
+
+Because of this, the only actual dependencies are [docker](https://docs.docker.com/get-docker/) and [docker-compose](https://docs.docker.com/compose/install/).
+
+## Setting up the project
+
+The first thing you need to do is fork the repository, and clone it in your local machine.
+
+Then you will have to follow these steps:
+
+* Copy all files with `.local.php.dist` extension from `config/autoload` by removing the dist extension.
+
+    For example the `common.local.php.dist` file should be copied as `common.local.php`.
+
+* Copy the file `docker-compose.override.yml.dist` by also removing the `dist` extension.
+* Start-up the project by running `docker-compose up`.
+
+    The first time this command is run, it will create several containers that are used during development, and may tike some time.
+
+    It will also create some empty databases and install the project dependencies with composer.
+
+* Run `./indocker bin/cli db:create` to create an empty database.
+* Run `./indocker bin/cli db:migrate` to get database migrations up to date.
+* Run `./indocker bin/cli api-key:generate` to get your first API key generated.
+
+Once you finish this, you will have the project exposed in ports `8080` through nginx+php-fpm and `8000` through swoole.
+
+> Note: The `indocker` shell script is a helper used to run commands inside the main docker container.
+
+## Running code checks
+
+* Run `./indocker composer cs` to check coding styles are fulfilled.
+* Run `./indocker composer cs:fix` to fix coding styles (some may not be fixeable from the CLI)
+* Run `./indocker composer stan` to check the code with phpstan. This tool is the closest you have to "compile" PHP and verify everything would work as expected.
+* Run `./indocker composer test:unit` to run the unit tests.
+* Run `./indocker composer test:db` to run integration tests with the database.
+
+    This command runs the same test suite against all supported database engines. If you just want to run one of them, you can add one of `:sqlite`, `:mysql`, `:maria`, `:postgres`, `:mssql` to the command to run just one of them.
+    
+    For example, `test:db:postgres`.
+
+* Run `./indocker composer test:api` to run API E2E tests. For these, the MySQL database engine is used.
+
+> Note: Due to some limitations in the tooling used by shlink, the testing databases need to exist first, both for db and api tests (except sqlite).
+>
+> However, they just need to be created empty, with no tables. Also, once created, they are automatically reset before each execution.
+>
+> The testing database is always called `shlink_testing`. You can create it using the database client of your choice. [DBeaver](https://dbeaver.io/) is a good multi-platform desktop database client which supports all the engines supported by shlink.
+
+## Pull request process