Document in client README that only CORS supported services run in the browser

[trivikr]

Describe the issue with documentation
There are several CORS issues reported on GitHub https://github.com/aws/aws-sdk-js-v3/search?q=cors+is%3Aissue&type=Issues
It would be awesome if we call out in client README about CORS support is managed by services.

Additional context
Refs: #1907 (comment)

[techthumb]

Hi,

Checking to understand if there will always be a CORS limitation.

Are there any workarounds that the team is considering?

Newer browsers are partitioning cookie stores for each originating domain.
Perhaps opening up CORS isn't that insecure any more?

Thanks

[github-actions]

Greetings! We’re closing this issue because it has been open a long time and hasn’t been updated in a while and may not be getting the attention it deserves. We encourage you to check if this is still an issue in the latest release and if you find that this is still a problem, please feel free to comment or open a new issue.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs and link to relevant comments in this thread.