Add example(s) with external stores?
I would love to get this working for my case, but I find the following part of the readme extremely cryptic:

> You can store the session content in external stores (Redis, MongoDB or other DBs) by passing options.store with three methods (these need to be async functions):
> 
>     get(key, maxAge, { rolling }): get session object by key
>     set(key, sess, maxAge, { rolling, changed }): set session object for key, with a maxAge (in ms)
>     destroy(key): destroy session for key

- How to pass "options.store with three methods"?
- On the `set` method, what is the `sess` parameter?

Would anyone be kind enough to add an example of the above (especially using redis)?
I searched everywhere and it is still a mystery to me how to get it working...

Thanks!!
Daniel
