documenting behavior when used with Base.Threads

Adding a note to the docs on loss of performance when Octavian is combined with Base.Threads.

Author: jlchan

docs/src/index.md

@@ -25,3 +25,9 @@ In general:
 - Octavian has the fastest CPU performance.
 - GemmKernels has the fastest GPU performance.
 - Tullio is the most flexible.
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+!!! note
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+    Octavian's tasks can interfere with tasks spawned by Base.Threads, resulting in much slower execution times when used together. This can be avoided by using threading utilities from [CheapThreads](https://github.com/JuliaSIMD/CheapThreads.jl) or [LoopVectorization](https://github.com/JuliaSIMD/LoopVectorization.jl/) instead. See this [Discourse post](https://discourse.julialang.org/t/odd-benchmarktools-timings-using-threads-and-octavian/59838) for more information.
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