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

[Fangrui Song 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
+the ``-flto`` variants. Without the ``-flto`` flag, ``-ffat-lto-objects`` has
+no effect.
+
+Compile an object file using FatLTO:
+
+.. code-block:: console
+
+   $ clang -flto -ffat-lto-objects example.c -o example.o
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MaskRay wrote:

Missing `-c`

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