Add an index to the docs

docs/index.md

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+    <img src="https://avatars2.githubusercontent.com/u/20600343" /><br/>
+</p>
+
+<h1 align="center">SymfonyExtension</h1>
+
+This Behat extension provides an integration with Symfony (both `^3.4` and `^4.1`) and Mink driver for Symfony application.
+
+It allows for:
+ 
+ * defining your contexts as regular Symfony services
+ 
+ * autowiring and autoconfiguring your contexts
+ 
+ * testing your Symfony application without having to set up a server
+ 
+## Documentation
+
+ * [Installation](docs/installation.md)
+ * [Usage](docs/usage.md)
+ * [Mink integration](docs/mink_integration.md)
+ * [Behat/Symfony2Extension differences](docs/bs2e_differences.md)
+ * [Configuration reference](docs/configuration_reference.md)
+ 
+## License
+
+This extension is completely free and released under permissive [MIT license](LICENSE).
+
+## Authors
+
+It is originally created by [Kamil Kokot](https://github.com/pamil). 
+See the list of [all contributors](https://github.com/FriendsOfBehat/SymfonyExtension/graphs/contributors).