Create README.md

Add README.md to Symfony2 plugin

plugins/symfony2/README.md

+# Symfony2
+
+This plugin provides completion for [symfony2](https://symfony.com/), as well as aliases for frequent composer commands.
+
+To use it add composer to the plugins array in your zshrc file.
+
+```bash
+plugins=(... symfony2)
+```
+
+## Aliases
+
+| Alias         | Command                      | Description                   |
+|---------------|------------------------------|-------------------------------|
+| `sf`          | php app/console              | Start the symfony console     |
+| `sfcl`        | sf cache:clear               | Clear the cache               |
+| `sfsr`        | sf server:run                | Run the dev server            |
+| `sfcw`        | sf cache:warmup              | Use the Bundles warmer        |
+| `sfroute`     | sf debug:router              | Show the different routes     |
+| `sfcontainer` | sf debug:contaner            | List the different services   |
+| `sfgb`        | sf generate:bundle           | Generate a bundle             |
+| `sfge`        | sf doctrine:generate:entity  | Generate an entity            |
+| `sfsu`        | sf doctrine:schema:update    | Update the schema in Database |
+| `sfdev`       | sf --env=dev                 | Update environment to `dev`   |
+| `sfprod`      | sf --env=prod                | Update environment to `prod`  |