Add more information on SIMD (#1138)

arrow/CONTRIBUTING.md

@@ -99,7 +99,15 @@ The arrow format declares a IPC protocol, which this crate supports. IPC is equi
 
 #### SIMD
 
-The API provided by the `packed_simd` library is currently `unsafe`. However, SIMD offers a significant performance improvement over non-SIMD operations.
+The API provided by the [packed_simd_2](https://docs.rs/packed_simd_2/latest/packed_simd_2/) crate is currently `unsafe`. However,
+SIMD offers a significant performance improvement over non-SIMD operations. A related crate in development is
+[portable-simd](https://rust-lang.github.io/portable-simd/core_simd/) which has a nice
+[beginners guide](https://github.com/rust-lang/portable-simd/blob/master/beginners-guide.md). These crates provide the ability
+for code on x86 and ARM architectures to use some of the available parallel register operations. As an example if two arrays
+of numbers are added, [1,2,3,4] + [5,6,7,8], rather than using four instructions to add each of the elements of the arrays,
+one instruction can be used to all all four elements at the same time, which leads to improved time to solution. SIMD instructions
+are typically most effective when data is aligned to allow a single load instruction to bring multiple consecutive data elements
+to the registers, before use of a SIMD instruction.
 
 #### Performance
 

arrow/README.md

@@ -40,8 +40,8 @@ The arrow crate provides the following features which may be enabled:
 - `prettyprint` - support for formatting record batches as textual columns
 - `js` - support for building arrow for WebAssembly / JavaScript
 - `simd` - (_Requires Nightly Rust_) alternate optimized
-  implementations of some [compute](https://github.com/apache/arrow/tree/master/rust/arrow/src/compute)
-  kernels using explicit SIMD processor intrinsics.
+  implementations of some [compute](https://github.com/apache/arrow-rs/tree/master/arrow/src/compute/kernels)
+  kernels using explicit SIMD instructions available through [packed_simd_2](https://docs.rs/packed_simd_2/latest/packed_simd_2/).
 - `chrono-tz` - support of parsing timezone using [chrono-tz](https://docs.rs/chrono-tz/0.6.0/chrono_tz/)
 
 ## Safety