[SIMD] write down the optimization and hotspot analysis

[huningxin]

start with v_dotprod.

[terfendail]

I've discussed the place to put this analysis with the team. It was decided to put the analysis to GSoC related PR either as a comment or as an attached document(whatever is more convenient). There still is a discussion about a unified place to retain all results and/or reports of this GSoC. But anyway if all results of the project are stored as a PR we will be able to move them to appropriate places later.

[huningxin]

It sounds good, thanks @terfendail . @Wenzhao-Xiang , please follow that. I think we need to move those info to your PR, including

• results, as [Doc] add the SIMD and threads+SIMD data of perf tests #300
• analysis of the results and related emscripten/v8 issues, like [SIMD] optimize the resize kernel #284 [SIMD] optimize the cvtColor kernel #301