add note about relevence of sysimg in 1.9 era

[sjkelly]

I did this benchmark on master (target 1.11), and got an end-to-end time of 0.8 seconds, so I think a note is appropriate here for newcomers about the relevance of sysimgs post-1.9.

[codecov[bot]]

Codecov Report

Merging #834 (029963a) into master (b4045fa) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #834   +/-   ##
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  Coverage   84.03%   84.03%           
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  Files           3        3           
  Lines         714      714           
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  Hits          600      600           
  Misses        114      114