Make a note that createStore mutates its args

[jb-1980]

I spent a few hours trying to track down a bug. And I was convinced that createStore must have been caching my results across re-renders. Then I realized the cached value was the default state value I passed as an argument when initializing the store. I think it would be nice to have a note about this in the documentation.

I made an example with a fork in Codesandbox. I am not a word smith, so any suggested improvements would be welcome.

If you agree, I would be happy to push up the changes and make a PR.