docs: make sure xhprof is installed

[phh]

Since this package uses the Spiral profiler package this package might specify the same requirements.

It might be obvious that you need Xhprof before this package can run, however, by just installing the package without the extension you just get an error:

Undefined constant "SpiralPackages\Profiler\Driver\XHPROF_FLAGS_MEMORY"

I've also decided not to link to Uprofiler or Tideways as these are both archived.

[maantje]

Hey @phh,

Thank you for clarifying the Xhprof requirement in the README!