Add intro, getting started and samples sections

guide/index.md

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+# Introduction
+
+## What's OpenIddict?
+
+OpenIddict aims at providing a **simple and easy-to-use solution** to implement an **OpenID Connect server in any ASP.NET Core 1.x or 2.x application**.
+
+OpenIddict is based on
+**[AspNet.Security.OpenIdConnect.Server (codenamed ASOS)](https://github.com/aspnet-contrib/AspNet.Security.OpenIdConnect.Server)** to control the OpenID Connect authentication flow and can be used with any membership stack, **including [ASP.NET Core Identity](https://github.com/aspnet/Identity)**.
+
+OpenIddict fully supports the **[code/implicit/hybrid flows](http://openid.net/specs/openid-connect-core-1_0.html)** and the **[client credentials/resource owner password grants](https://tools.ietf.org/html/rfc6749)**. You can also create your own custom grant types.
+
+Note: OpenIddict natively supports **[Entity Framework Core](https://github.com/aspnet/EntityFramework)** and **[Entity Framework 6](https://github.com/aspnet/EntityFramework6)** out-of-the-box, but you can also provide your own stores.
+
+> Note: **the OpenIddict 2.x packages are only compatible with ASP.NET Core 2.x**.
+> If your application targets ASP.NET Core 1.x, use the OpenIddict 1.x packages.
+
+## Why an OpenID Connect server?
+
+Adding an OpenID Connect server to your application **allows you to support token authentication**.
+It also allows you to manage all your users using local password or an external identity provider
+(e.g. Facebook or Google) for all your applications in one central place,
+with the power to control who can access your API and the information that is exposed to each client.