Add missing documentation about the Subscribe API

[jarifibrahim]

The subscribe API
https://github.com/dgraph-io/badger/blob/f50343ff404d8198df6dc83755ec2eab863d5ff2/db.go#L1510-L1516

was added in badger via 26128a7 but there is no documentation about it. We should also have an example of how to use the Subscribe API.

https://github.com/dgraph-io/badger/blob/f50343ff404d8198df6dc83755ec2eab863d5ff2/db_test.go#L1939-L1948 can be used for inspiration.

[jarifibrahim]

cc @balajijinnah

[kaustubhmallik]

@jarifibrahim is this still available to grab?

[kaustubhmallik]

@jarifibrahim By documentation, you mean adding explanations and usage (code example) in the README directly, correct? As far as I have understood, this feature enables you to add a post-update behaviour (callback function) for certain set of keys, configured using the key's prefix.

Also, as you said, the example can be directly quoted from the test you have written.

[minhaj-shakeel]

Github issues have been deprecated.
This issue has been moved to discuss. You can follow the conversation there and also subscribe to updates by changing your notification preferences.