Add info about embedding interfaces into structs

[itaranto]

Currently, only struct on struct or interface on interface embedding is explained in OOP.md.

It would be nice to have an explanation about embedding interfaces into structs.

Here's a nice example:
https://github.com/dapr/components-contrib/blob/master/state/store.go#L51

In this example the DefaultBulkStore struct embeds the Store interface, DefaultBulkStore the uses the embedded Store interface to also implement BulkStore.