documenting behavior when used with Base.Threads

[jlchan]

Adding a note to the docs on loss of performance when Octavian is combined with Base.Threads. CheapThreads and LoopVectorization are suggested as alternatives.

[chriselrod]

We could add an explanation of why, but this is fine. Looks good to me.
Maybe I should make it so that tests don't rerun if no files in src or test were actually updated.

[codecov]

Codecov Report

Merging #80 (276916a) into master (49673bc) will decrease coverage by 1.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   86.54%   84.60%   -1.95%     
==========================================
  Files          10       10              
  Lines         565      565              
==========================================
- Hits          489      478      -11     
- Misses         76       87      +11     

Impacted Files	Coverage Δ
src/global_constants.jl	46.66% <0.00%> (-20.00%)	⬇️
src/matmul.jl	88.67% <0.00%> (-1.14%)	⬇️
src/block_sizes.jl	96.61% <0.00%> (+1.69%)	⬆️

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[jlchan]

We could add an explanation of why, but this is fine

Oops - just saw this. I figured the explanation was partially in the Discourse post, so I tried to keep the note concise.