documentation of requirements for "subscriptions" collection

[damsleth]

The subscriptions collections has a few requirements that must be followed to be able to add subscriptions with did-cli

• The collection is case sensitive, i.e. Subscriptions and SUBSCRIPTIONS won't work, only subscriptions. Also plural.
• The shard key must be one of the root properties of the subscription document added by did-cli, either name or db should do.

This should be clearly documented, ideally in the cli itself when you run did subscription add

[damsleth]

auto-creating subscriptions-collection on did subscription add first run (see #15 ) is the best solution IMO