Provide Microsoft.AspNetCore.DataProtection.StackExchangeRedis package README

[MackinnonBuck]

Contributes to #48392

[amcasey]

Thanks! This is awesome!

I wonder if it would be worthwhile to include this blurb or link: https://learn.microsoft.com/en-us/aspnet/core/security/data-protection/configuration/overview?view=aspnetcore-8.0#persisting-keys-with-redis. @mgravell might know more about whether it applies to this particular package.

[mgravell]

Yes, I think the warning about persistence is important enough to include at this level; the warning could (as suggested) be lifted direct from the link provided - just that one line (it is a good line!).

[MackinnonBuck]

Thanks, @mgravell and @amcasey! I've just updated the PR to include that warning.