docs: Add Settings documentation

docs/settings.md

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+# Settings
+
+There are many settings that can be set for Validation layers. This is a brief overview how to use them.
+
+There are 4 ways to set settings: `vkconfig`, `application defined`, `vk_layer_settings.txt`, `environment variables`
+
+## VkConfig
+
+We suggest people to use [VkConfig](https://www.lunarg.com/introducing-the-new-vulkan-configurator-vkconfig/).
+
+The GUI comes with the SDK, and takes the `VkLayer_khronos_validation.json` file and does **everything** for you!
+
+## Application Defined
+
+The application can now use the `VK_EXT_layer_settings` extension to do everything at `vkCreateInstance` time. (Don't worry, we implement the extension, so it will be supported 100% of the time!).
+
+```c++
+// Example how to turn on verbose mode for DebugPrintf
+const VkBool32 verbose_value = true;
+const VkLayerSettingEXT layer_setting = {"VK_LAYER_KHRONOS_validation", "printf_verbose", VK_LAYER_SETTING_TYPE_BOOL32_EXT, 1, &verbose_value};
+VkLayerSettingsCreateInfoEXT layer_settings_create_info = {VK_STRUCTURE_TYPE_LAYER_SETTINGS_CREATE_INFO_EXT, nullptr, 1, &layer_setting};
+
+VkInstanceCreateInfo instance_ci = GetYourCreateInfo();
+instance_ci.pNext = &layer_settings_create_info;
+```
+
+## vk_layer_settings.txt
+
+There is info [elsewhere](https://vulkan.lunarg.com/doc/view/latest/windows/layer_configuration.html) to describe this file, but the short answer is to go
+
+```
+# windows
+set VK_LAYER_SETTINGS_PATH=C:\path\to\vk_layer_settings.txt
+
+# linux
+export VK_LAYER_SETTINGS_PATH=/path/to/vk_layer_settings.txt
+```
+
+and it will set things for you there. We have a [default example](../layers/vk_layer_settings.txt) file you can start with
+
+## Environment Variables
+
+This is done for us via the `vkuCreateLayerSettingSet` call in the [Vulkan-Utility-Libraries](https://github.com/KhronosGroup/Vulkan-Utility-Libraries/).
+
+As an example, in our `VkLayer_khronos_validation.json` file you will find something like `"key": "message_id_filter",`.
+
+From here you just need to adjust it the naming and prefix depending on your platform:
+
+```
+# Windows
+set VK_LAYER_MESSAGE_ID_FILTER=VUID-VkInstanceCreateInfo-pNext-pNext
+
+# Linux
+export VK_LAYER_MESSAGE_ID_FILTER=VUID-VkInstanceCreateInfo-pNext-pNext
+
+# Android
+adb setprop debug.vulkan.khronos_validation.message_id_filter=VUID-VkInstanceCreateInfo-pNext-pNext
+```
+
+## Finding available settings
+
+How I suggest finding them:
+
+1. Check VkConfig
+2. View `VkLayer_khronos_validation.json` (it is where we define them all)
+3. In [layer_options.cpp](../layers/layer_options.cpp) (it is where we parse and set the settings)
+
+## Legacy
+
+This is only here to document the legacy of how we got to this situation.
+
+Long ago validation created its own system to parse environmental variables as well as the `vk_layer_settings.txt` flow.
+
+Then we created `VK_EXT_validation_features` as way to set things at `vkCreateInstance` time. `VK_EXT_validation_flags` was also created after with the same goals in mind.
+
+As more and more layers basically needed to do what Validation Layers were doing, we just ended up creating `VK_EXT_layer_settings` as a final solution. This extension has been the way forward to a better system of creating settings for layers.
+
+Unfortunately, we still support some legacy names, so this prevents us from making everything consistent.