Overhaul README with collapsible sections and add section discussing structs

[MasonProtter]

The readme in its current form was a bit sprawling and hard to manage. I made sections of it collapsible so it is easier to navigate and get an overview. An alternative approach would be a table of contents.

I also added a section called "Dealing with structs" where I recommend StructArrays.jl for kernels involving unsupported number types and demo it with that gemm kernel I posted on discourse.

[codecov-io]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
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  Coverage   84.82%   84.82%           
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  Files          10       10           
  Lines        1285     1285           
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  Hits         1090     1090           
  Misses        195      195

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[chriselrod]

Thanks, this great!

I'll fix these typos on line 176, dealing with structs next time I make a commit:
"arrays conatining structs"
"currently reccomended that"

[MasonProtter]

I'll fix these typos on line 176, dealing with structs next time I make a commit:
"arrays conatining structs"
"currently reccomended that"

Whoops, sorry about that.