Documenting the story with `async` handling

[czocher]

Use Case:

Using sled with async is not well documented.

Proposed Change:

Sled should have a readme section or (better) an example of handling it in an async case when the database is not in-memory.

Who Benefits From The Change(s)?

People using sled as storage with webservices, which are mostly asynchronous at this moment.

Alternative Approaches

Add some special in-code handling of async, but that's unnecessary in my opinion.

[flipchan]

@czocher any updates on this?

[czocher]

@flipchan sled seems mostly unmaintained, the authors moved to a different kv store implementation. I urge you to reconsider choosing this library for this reason alone.