Add documentation

[RazrFalcon]

It would be great to have a documentation, especially Intel-like: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#expand=3333,100,100&text=_mm512_add_epi32

FOR j := 0 to 15
	i := j*32
	dst[i+31:i] := a[i+31:i] + b[i+31:i]
ENDFOR
dst[MAX:512] := 0

[Lokathor]

Yeah, docs would be good.

Most functions can probably just be a note like "lanewise add" or "lanewise sqrt" or similar.

I want to avoid excessive rustdoc example code because that makes the test take far longer to run through.

I'd accept PRs with docs, but probably won't end up doing this myself because the Portable SIMD project group has officially begun, so most of my energy will go there. Ideally what comes out of that project will totally supercede this crate without any loss of ergonomics.