Add note about Apple Silicon support

[rovo79]

@danthe3rd
Add Supported Platforms section to README
Add a new section called "Supported Platforms" before the Installation to set expectations around supported systems before users attempt installation.

Adding this section aims to reduce issues and questions about lack of Apple Silicon support. It makes the limitations explicit upfront, before users try installing.

What does this PR do?

Fixes #858 .

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[danthe3rd]

Thanks for putting a PR to fix this! I've added a few comments
Can we move this section after the "Installing xFormers" one?

[rovo79]

Makes sense to me.

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