Document easy way for Xeon processors to build

[snadrus]

No description provided.

[LexLuthr]

Looks good with one nit

[LexLuthr]

Suggested change

	3. Also for old processors, if Curio cannot start because the processor generation is too old, add the variable:\
	3. Also for old processors, if Curio cannot start because the processor generation is too old, add the variable and rebuild the binaries:\

[magik6k]

Can we auto-detect maybe?

Something like

ifeq ($(shell grep -q avx512f /proc/cpuinfo && echo yes),yes)
    export GOAMD64 := v3
else
    export GOAMD64 := v1
endif

[LexLuthr]

Auto detect might be a better way instead of docs.