Add graphql-pg-subscription package link (A pubsub class for postgres database interface)

[siamahnaf]

I recently work on graphql-pg-subscription package. I see you already add a package that name is graphql-postgres-subscription, which is not properly maintained. It has not typescript support, maxListener support and has large payload problem. So that's why I build this open source package for adding pubsub class with postgres interface.

I will happy if you add this package into your docs. Thanks Apollo Teams.

[apollo-cla]

@siamahnaf: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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