Merge "vulkan: combine doc with README.md file"

This subdirectory contains Android's Vulkan loader, as well as some Vulkan-related tools useful to platform developers.

## Documentation

The former contents of doc/implementors_guide/ are now at https://source.android.com/devices/graphics/implement-vulkan.

## Coding Style

We follow the [Chromium coding style](https://www.chromium.org/developers/coding-style) for naming and formatting, except with four-space indentation instead of two spaces. In general, any C++ features supported by the prebuilt platform toolchain are allowed.

We generate several parts of the loader and tools driectly from the Vulkan Registry (external/vulkan-headers/registry/vk.xml). Code generation must be done manually because the generator is not part of the platform toolchain (yet?). Files named `foo_gen.*` are generated by the code generator.

 To run the generator:

- Install Python3 (if not already installed)

- `$ ./<path to>/frameworks/native/vulkan/scripts/code_generator.py`

### Run The Code Generator

Install Python3 (if not already installed) and execute below:

`$ ./scripts/code_generator.py`

The former contents of implementors_guide/ are now at

https://source.android.com/devices/graphics/implement-vulkan