Redis: package README + config test

[renansj]

Summary

• Add a README for the AppRateLimiter.Redis package and include it in the nupkg (fixes the 'Readme missing' warning seen on publish).
• Add a test covering AddRedisRateLimiter(connectionString, keyPrefix): it registers a working store (the same path the sample app uses) and verifies the custom key prefix is applied.

Tested

• Verified locally that the Redis package now packs with README.md and MIT license in the nuspec.
• New test runs against real Redis in CI (SkippableFact, like the others).