Add a page explaining how to use custom entities with OpenIddict

[kevinchalet]

2 goals:

• Describe how to use the correct overloads when using a custom key type or custom entities derived from the default entities.

• Add a few hints about you can create custom stores when opting for custom entities that don't derive from the built-in ones.

(opened on behalf of Weston Weems)

[kevinchalet]

Describe how to use the correct overloads when using a custom key type or custom entities derived from the default entities.

That part is covered by https://github.com/openiddict/openiddict-documentation/commit/eb3fd55b65a82cadf163467cdde4b14121a74e99.

[kevinchalet]

https://github.com/openiddict/openiddict-documentation/commit/ffa662ad42029c721ff3cfefbdfb1269f65c13da added similar documentation for Entity Framework 6.x.

[CDian72]

Is it possible to place a sample for this topic ?