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 eb3fd55.

[kevinchalet]

ffa662a added similar documentation for Entity Framework 6.x.

[CDian72]

Is it possible to place a sample for this topic ?