[Documentation] gmock appears to be necessary for shaderc

[rubyFeedback]

Could the main README briefly mention that gmock is a dependency; and where
to get it? Ideally in the main README, or some FAQ entry on the wiki or something
like that. That way people who want to get shaderc for the vulkan-releated stuff,
know where to go to next. Thanks for reading.

[ben-clayton]

The step to fetch the dependency is listed in the root README:

https://github.com/google/shaderc?tab=readme-ov-file#getting-and-building-shaderc

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1. Check out the source code:

git clone https://github.com/google/shaderc $SOURCE_DIR
cd $SOURCE_DIR
./utils/git-sync-deps

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You need to call ./utils/git-sync-deps.