* Add POWER8 VSX toolchains

POWER8, though slower than POWER9, is still used in the wild; these
toolchains should still be much faster on POWER8 than POWER8 without VSX
optimizations.

* VSX toolchains: set -cpu arg in QEMU CI tests

* Add POWER9 VSX toolchains

Translating x86_64 SSE to ppc64le VSX intrinsics yields a quite large
speedup on POWER9. See this article for background:

https://www.talospace.com/2019/07/easier-power-vectorizing-for-fun-and.html

* Add power9le docs

* power9le clang toolchain: Document Clang 13+ requirement

---------

Co-authored-by: Jeremy Rand <jeremyrand@danwin1210.de>

1. The correctly link is `how-to-build#build-for-linux` not `#pass-for-linux`, and so on other `#`
2. Add compatibility introduction for Raspberry Pi 4 (build pass)

* Dedicated to coloring black and white photographs.

* Update README.md

* Update how-to-build.md

* Update README.md

* Update README.md

* Update ncnn-android-scrfd link and demo picture

* Update ncnn-android-scrfd link and demo screenshot

Signed-off-by: daquexian <daquexian566@gmail.com>

* Add [ MacOS ->  apple-cpu/apple-gpu] into supported platform matrix
 * Add [ mac ->  apple] into 'device support vulkan' list

Co-authored-by: meow <826150766@qq.com>

* add vs2015 ci
* build x86 binary

* codecov for arm32 aarch64, vs2017 ci

* vs2017 default to x86 even on x64 host

Update README.md for Netron.app

* Add readme for keras2ncnn

* Add supported model variants

* Fix supported model variants

* Improve build instructions

* Merge Linux stuff into one section.
* Provide better info on dependencies
* Provide easier to use instruction for Debian / Ubuntu for native and cross-compile builds
* Capitalize Sentence and put proper punctation.
* Use backticks where appropiate
* Move Hisilicon instruction together with Arm crosscompiling
* Add information how to verify the Vulkan installation on Linux
* Change how to launch examples so it is easier (no copying needed or external resources)
* Add info about benchmark and how to test it on GPU or CPU
* Use `$(nproc)` where appropriate.
* Put some paths in quotes, in case they contain spaces or other weird characters (coma, semicolon, colon, etc).
* Remove some extremely obvious pieces that every developer usually knows.
* Use newer Vulkan SDK in instructions for Linux
* Get rid of `<ncnn-root-dir>`, and just use `ncnn`. It is what git clone does. And any developer will know how to change it if they choose to do it differently.

* Update README.md

* Update how-to-build.md

Co-authored-by: nihui <shuizhuyuanluo@126.com>