[Doc]add doc for enable SIMD need cargo nightly

[Ted-Jiang]

Which issue does this PR close?

I have tested the simd flag in my local. But the doc does not say using nightly. got

error[E0554]: `#![feature]` may not be used on the stable release channel
   --> /Users/yangjiang/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd_2-0.3.7/src/lib.rs:214:1
    |
214 | / #![feature(
215 | |     adt_const_params,
216 | |     repr_simd,
217 | |     rustc_attrs,
...   |
226 | |     custom_inner_attributes,
227 | | )]
    | |__^

Closes #.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?