Add supported runtimes section and Cloudflare import note

[RockerFlower]

This pull request adds two pieces of information to the Remix Auth README:

• A "Supported runtimes" section that clearly shows which environments Remix Auth supports. This addition helps users quickly determine if Remix Auth is compatible with their deployment environment.
• A note about importing from @remix-run/cloudflare when deploying to Cloudflare. This information is crucial for users deploying to Cloudflare, as it prevents potential import errors.