Improve CONTRIBUTING guide

Also:

- update the docker-compose to use Alpine image (lighter)
- update the PHP Dockerfile to use Imagick instead of GD (for GIF support when downloading images) and add support for SQLite

.github/CONTRIBUTING.md

@@ -1,11 +1,41 @@
 # How to contribute
 
+## Test it locally
+
+### Using Docker
+
+- Clone the repository
+- Ensure your Docker daemon is running
+- Launch `docker-compose up`
+
+You'll then have:
+- a web server (nginx)
+- a PHP daemon (using FPM)
+- a Redis database (to handle imports)
+- a SQLite database to store articles
+
+You can now access your wallabag instance using that url: `http://127.0.0.1:8000`
+
+If you want to test using an other database than SQLite, uncomment the `postgres` or `mariadb` code from the `docker-compose.yml` file at the root of the repo. Also uncomment related line in the `php` section so the database will be linked to your PHP instance.
+
+### Using your own PHP server
+
+- Ensure you are running PHP > 7.1.
+- Clone the repository
+- Launch `composer install`
+- If you got some errors, fix them (they might be related to some missing PHP extension from your machine)
+- Then `php bin/console wallabag:install`
+- If you got some errors, fix them (they might be related to some missing PHP extension from your machine)
+- Run `php bin/console server:run`
+
+You can now access your wallabag instance using that url: `http://127.0.0.1:8000`
+
 ## You found a bug
 Please [open a new issue](https://github.com/wallabag/wallabag/issues/new).
 
 To fix the bug quickly, we need some infos: please answer to the questions in the issue form.
 
-If you have the skills, look for errors into php, server and application (see `var/logs`) logs.
+If you have the skills, look for errors into PHP, server and application logs (see `var/logs`).
 
 Note : If you have large portions of text, use [Github's Gist service](https://gist.github.com/) or other pastebin-like.