start on SIMD documentation

[bearophile]

Peter Alexander:

> I'm not so sure on using names like 
> int4 for vectors. You see things like int32 a lot to mean a 32-bit 
> integer, or int8 to mean an 8-bit integer. Using this notation for 
> vectors may be confusing.
> 
> As for what to change it to, I don't really care. int4v, vec_int4, 
> int_vector4, anything. It doesn't matter.

It's not confusing for D programmers. But if you want to be on the safe side then int4v or vint4 sound acceptable.

Bye,
bearophile