Document subscribing to events and entity states

[armiol]

Until recently Stand API allowed to subscribe to the Entity state updates only. E.g.

"As a user I was able to subscribe to all changes of `Project` Aggregate with ID = 42."

However, it's been already a couple of months Spine users are able to subscribe to domain events as well. E.g.

"As a user I am able to subscribe to `TaskUpdated` events once they occur."

We need to mention that in a read-side API documentation. And have an code example illustrating the feature.

[meizabel]

I have a couple of questions related to the issue:

1. Should we mention it in Model Definition Guide or somewhere else on the site? Please clarify.
2. Also, could you please provide a code sample for the feature?

[meizabel]

@armiol
Could you please respond to my questions above? Thanks in advance!

[armiol]

@meizabel please find my answers below.

1. Yes, the Model Definition Guide should have a section called "Subscriptions" below the "Queries". In scope of this section, we are going to speak of Stand and its functionality, including an ability for API clients to subscribe to both entity state changes and domain events.
2. The code sample for the feature is basically the same as the code of subscribing to entity state via Stand. Once we have an illustrative example of subscriptions, we will have the code sample requested as well.

[meizabel]

@armiol
Do we have illustrative examples of subscriptions now?

[armiol]

We need to have a code sample in the Stand documentation. And have an article on subscriptions from the client-side in the documentation.