Basic readme telling how to select symfony version (#23)

* Basic readme telling how to select symfony version

* Describe installing symfony with custom stability

* Update README.md

Author: mateuszsip

README.md

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+# Symfony Docker
+
+A [Docker](https://www.docker.com/)-based installer and runtime for the [Symfony](https://symfony.com) web framework.
+
+## Getting Started
+
+1. Run `docker-compose up` (the logs will be displayed in the current shell)
+2. Open `https://localhost` in your favorite web browser and [accept the auto-generated TLS certificate](https://stackoverflow.com/a/15076602/1352334)
+3. **Enjoy!**
+
+## Selecting a Specific Symfony Version
+
+Use the `SYMFONY_VERSION` environment variable to select a specific Symfony version.
+
+For instance, use the following command to install Symfony 3.4:
+
+`SYMFONY_VERSION=3.4 docker-compose up --build`
+
+To install a non-stable version of Symfony, use the `STABILITY` environment variable during the build.
+The value must be a valid [a valid Composer stability option](https://getcomposer.org/doc/04-schema.md#minimum-stability)) .
+
+For instance, use the following command to use the `master` branch of Symfony:
+
+`STABILITY=dev docker-compose up --build`
+
+## Credits
+
+Created by [Kévin Dunglas](https://dunglas.fr) and sponsored by [Les-Tilleuls.coop](https://les-tilleuls.coop).