Subscriptions docs suggestions

[Urigo]

Adding some suggestions to the subscriptions section:

As mentioned in this comment, the graphql-subscriptions library is not maintained (reference) I've made a change to recommend another library for the solution.

In order to move this our of draft, @enisdenjo @ardatan I believe we'll need to publish a README specifically for that library.

I've also changed the comment about WebSockets and SSE, I'm not sure if we want to say that most people use WebSockets?

[benjie]

Is it unmaintained? According to npm its last release was 7 months ago:

• graphql-subscriptions on npm
• 3.0.0 release notes

[enisdenjo]

v3 was pending release for like 3 years... Issues are also largely unanswered https://github.com/apollographql/graphql-subscriptions/issues.