libvmaf/README: mention SIMD is x86

Author: kylophone

README.md

@@ -11,7 +11,7 @@ Also included in `libvmaf` are implementations of several other metrics: PSNR, P
 
 ## News
 
-- (2020-12-3) We are releasing `libvmaf v2.0.0`. It has a new fixed-point and SIMD-optimized implementation that achieves 2x speed up compared to the previous floating-point version. It also has a [new API](libvmaf/README.md) that is more flexible and extensible.
+- (2020-12-3) We are releasing `libvmaf v2.0.0`. It has a new fixed-point and x86 SIMD-optimized (AVX2, AVX-512) implementation that achieves 2x speed up compared to the previous floating-point version. It also has a [new API](libvmaf/README.md) that is more flexible and extensible.
 - (2020-7-13) We have created a [memo](https://docs.google.com/document/d/1dJczEhXO0MZjBSNyKmd3ARiCTdFVMNPBykH4_HMPoyY/edit?usp=sharing) to share our thoughts on VMAF's property in the presence of image enhancement operations, its impact on codec evaluation, and our solutions.
 - (2020-2-27) We have changed VMAF's license from Apache 2.0 to [BSD+Patent](https://opensource.org/licenses/BSDplusPatent), a more permissive license compared to Apache that also includes an express patent grant.