Add deprecation notice to README and JSDoc

[EskiMojo14]

fixes #183

Redux Mock Store still has a lot of usage in the ecosystem, but the Redux team believes there are better approaches to testing a store. Similar to our deprecation of createStore, deprecating this package/function will reach people who may not have seen our updated testing guidance.

A legacy_configureStore export without the deprecation notice is available.

The JSDoc will also need updating over on the DefinitelyTyped side, but we'll do that once we've agreed how to do it here.

[EskiMojo14]

Hi @arnaudbenard @dmitry-zaets :)

Would either of you be able and willing to add me as a maintainer to the npm package? My npm username is eskimojo. (you should be able to see me as a member of the @reduxjs org)

Happy to discuss this deprecation with either of you if you have any concerns!

cc @jednano as your fork has some traction 👋🏻

[dmitry-zaets]

hi @EskiMojo14.
Sure, makes sense. Let's proceed with this.
Could you please re-format PR to follow the same rules for the code as it was following? (Single quotes and no ';' in the end of line)

[EskiMojo14]

@dmitry-zaets sure - I've added a quick prettier config for people (like myself) who have auto formatting on save.

[dmitry-zaets]

@EskiMojo14 Added you as a maintained to the npm package. Feel free to publish a new version

[EskiMojo14]

@dmitry-zaets published as 1.5.5, thank you! I'll get a release note written up soon.

edit - https://github.com/reduxjs/redux-mock-store/releases/tag/v1.5.5