Clarify user info storage in README

[mattmazzola]

I thought there was some confusing wording in the readme about using cookies and localstorage.

cookie in local storage with your user's information

https://github.com/Swizec/useAuth/blame/master/README.md#L126

1. I didn't think it was possible to store a "cookie in localstorage". You either store info in a cookie or store info in local storage but not a cookie in local storage.

2. It says "user information". If this is non-secret stuff from the ID token like name I suppose that's ok but as I understood Auth0 was following all the best practices and not storage sensitive / secrete info in localStorage since it could be compromised by 3rd party scripts. Maybe clarify what this user information is so users don't get worried and think this library is risky.