Document how to set up read-only mounted shared artifact cache

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In normal functioning, QBT will extract an artifact from the cache each time it is needed during a build. This means if you build 100 packages, and each depends upon package Foo, then you extract 100 temporary copies of package Foo.

For performance reasons, it is possible to instruct QBT to extract an artifact only once and instead symlink it in each time it is used, but this is dangerous because if any build misbehaves, it might alter that artifact, poisoning your build's own cache.

We have developed a "best of both worlds" setup to address this but it is complex. Most users won't care to do this but for build nodes or places where performance is sensitive, you can instruct QBT to symlink and use a read-only bind mount to only access the artifacts via a read-only mountpoint during the package builds. The code to do this is all there but we should document how to set this up.