Add provider documentation index

Add an initial documentation index for the providers for aspnet-contrib#459.

Author: martincostello

docs/README.md

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+# OAuth Provider Documentation
+
+This document contains some introductory information about how to integrate the
+OAuth providers in this repository into an ASP.NET Core application.
+
+It assumes a general familiarity with ASP.NET Core and authorization, and is
+intended to demonstrate how to configure it into your application, not how it
+works internally. Further integration details for a given provider can be found
+in the services' own documentation, which are linked to in the main [README](https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers#providers "Table of OAuth providers").
+
+## Generic Documentation
+
+Most of the OAuth providers in this repository can be configured by just
+specifying two settings: `ClientId` and `ClientSecret`.
+
+Let's use the Slack provider as an example:
+
+```csharp
+services.AddAuthentication(options => /* Auth configuration */)
+        .AddSlack(options =>
+        {
+            options.ClientId = "my-client-id";
+            options.ClientSecret = "my-client-secret";
+        });
+```
+
+With just two settings, you can configure the Slack provider to authenticate users.
+
+The providers all follow a convention, so you just need to add the appropriate
+NuGet package reference and replace the word `Slack` in the method name with the
+provider name you are integrating. So for Spotify, it would be `AddSpotify()`.
+
+## Provider-Specific Documentation
+
+Any providers listed here have additional configuration that is either required
+or optional beyond the standard built-in OAuth configuration settings.
+
+The table below lists all of the providers that provide additional configuration
+and a link to a provider-specific document for that provider. If the provider
+you are using is not listed, it does not have any specific documentation and is
+covered by the section above.
+
+| Provider | Required/Optional Settings | Documentation Link |
+|:-:|:-:|:-:|
+| Amazon | _Optional_ | [Documentation](amazon.md "Amazon provider documentation") |
+| Apple | **Required** | [Documentation](sign-in-with-apple.md "Apple provider documentation") |
+| BattleNet | | [Documentation](battlenet.md "BattleNet provider documentation") |
+| Bitbucket | | [Documentation](bitbucket.md "Bitbucket provider documentation") |
+| Discord | | [Documentation](discord.md "Discord provider documentation") |
+| EVEOnline | | [Documentation](eveonline.md "EVEOnline provider documentation") |
+| Foursquare | | [Documentation](foursquare.md "Foursquare provider documentation") |
+| GitHub | _Optional_ | [Documentation](github.md "GitHub provider documentation") |
+| Gitee | | [Documentation](gitee.md "Gitee provider documentation") |
+| Instagram | | [Documentation](instagram.md "Instagram provider documentation") |
+| LinkedIn | | [Documentation](linkedin.md "LinkedIn provider documentation") |
+| Odnoklassniki | | [Documentation](odnoklassniki.md "Odnoklassniki provider documentation") |
+| Okta | **Required** | [Documentation](okta.md "Okta provider documentation") |
+| Patreon | | [Documentation](patreon.md "Patreon provider documentation") |
+| QQ | | [Documentation](qq.md "QQ provider documentation") |
+| Reddit | | [Documentation](reddit.md "Reddit provider documentation") |
+| Salesforce | | [Documentation](salesforce.md "Salesforce provider documentation") |
+| StackExchange | | [Documentation](stackexchange.md "StackExchange provider documentation") |
+| SuperOffice | **Required** | [Documentation](superoffice.md "SuperOffice provider documentation") |
+| Trakt | | [Documentation](trakt.md "Trakt provider documentation") |
+| Twitch | | [Documentation](twitch.md "Twitch provider documentation") |
+| Vkontakte | | [Documentation](vkontakte.md "Vkontakte provider documentation") |
+| Weibo | | [Documentation](weibo.md "Weibo provider documentation") |