Add explanation on how to use the RISCV vector implementations to the README

[mr-c]

@eric900115 can you do this?

[eric900115]

Sure. I'll do this.

[mr-c]

This explanation should document the issues faced in #1184 and their solutions

[vhaudiquet]

Is this still open? It seems that the RISC-V documentation is still lacking...
I've stumbled upon programs using simde (WebKit), and had bugs were on riscv64 it included arm/neon.h headers. I first thought that it was a complete error, like a fallback to arm when arch != x86. I then realized that some of the RISC-V code uses arm/neon.h headers.

I'm not sure if you plan to rename those headers (if some are providing features for different architectures, why are they under an arch-specific folder?), but having a bit of documentation would be a good start, as I'm still not sure what is going on. It is not really relevant to the bug being fixed (which is already done), just trying to say that from the point of view of an external RISC-V developer, it's complicated to understand what is going on.

I say that knowing that RISC-V might not be your priority at all, and that it might be complicated to document something that is still evolving, so I don't expect a change, just wanted to ask what was the status 😃

Thanks for simde!