[llvm] [docs][NFC] Add usage instructions for FatLTO (PR #75853)

[Paul Kirth via llvm-commits]

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@@ -76,3 +76,40 @@ Supported File Formats
 The current implementation only supports ELF files. At time of writing, it is
 unclear if it will be useful to support other object file formats like ``COFF``
 or ``Mach-O``.
+
+Usage
+===========
+
+Users are expected to pass ``-ffat-lto-objects`` to clang in addition to one of
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ilovepi wrote:

done

https://github.com/llvm/llvm-project/pull/75853