Update readme: add redux-thunk example, add sandbox example

[Buggytheclown]

Great work, redux-optimistic-ui - is amazing!

I found it difficult to understand the documentation on first reading.

• Is middleware required?
• Is the increment counter of nextTransactionID ++ required?
• How will it look like in the real world?

Add Sandbox example - https://codesandbox.io/s/redux-optimistic-update-g0mrb Add real world example to the readme