Add multithreaded example

[quietlychris]

I've seen a couple questions pop up in the issues about using redb in a multi-threaded context or async-task context such as #555. I thought it might be useful to have an example of what that might look like. If anything's incorrect or there are changes that would make this more representative, just let me know!

[cberner]

I think the example would be better if you had some, say 8, threads performing reads, while a few, say 2, threads did the writes. One feature that's useful to demonstrate is that readers are not blocked by writers

[quietlychris]

Sorry for the delay! I've tried updating this to have 8 readers and writers; the println! statements seems a little messy, but not sure if there's a better way to demonstrate the behavior.