Update Install Instructions to include note on local accessing files

[renderbox]

This is something that tripped me up but would be a quick add to the readme when installing with Chrome (I installed via the web store).

When I tried install and use the Redux DevTools, it could not find my store. This was extremely frustrating until I realized that by default, Chrome does not give the plugin access to file based URLs (and results as ‘undefined’ in the console). After turning that on, it found my Redux store and was able to use the tool.

[jcready]

https://github.com/zalmoxisus/redux-devtools-extension/blob/master/docs/Troubleshooting.md#access-file-url-file