Update README.md

Author: lemire

README.md

@@ -267,6 +267,8 @@ You can print the content of a vector register like so:
 
 - Be careful: `Vector128.Shuffle` is not the same as `Ssse3.Shuffle` nor is  `Vector256.Shuffle` the same as `Avx2.Shuffle`. Prefer the latter.
 - Similarly `Vector128.Shuffle` is not the same as `AdvSimd.Arm64.VectorTableLookup`, use the latter.
+- `stackalloc` arrays should probably not be used in class instances.
+- In C#, `struct` might be preferable to `class` instances as it makes it clear that the data is thread local.
 
 ## More reading