Added more info for "Installer Types" values in Settings.md. (microsoft#6067)

The current documentation on the priority orders of installer types when
using `winget install` without having specified `--installer-type`, was
unfortunately pretty scarce. So I've filled in most of the gaps now.

Author: DandelionSprout

doc/Settings.md

@@ -8,6 +8,8 @@ Settings file is located in %LOCALAPPDATA%\Packages\Microsoft.DesktopAppInstalle
 
 If you are using the non-packaged WinGet version by building it from source code, the file will be located under %LOCALAPPDATA%\Microsoft\WinGet\Settings\settings.json
 
+If you are creating a settings file from scratch, make sure that the line `"$schema": "https://aka.ms/winget-settings.schema.json",` is in it.
+
 ## Source
 
 The `source` settings involve configuration to the WinGet source.
@@ -180,7 +182,11 @@ The `architectures` behavior affects what architectures will be selected when in
 
 ### Installer Types
 
-The `installerTypes` behavior affects what installer types will be selected when installing a package. The matching parameter is `--installer-type`.
+The `installerTypes` behavior affects what installer types will be selected when installing a package. It can also determine which type to install by default if a manifest has multiple types: The list is in priority order, with the first listed type being preferred over the others, and so on.  This is convenient for users who for instance prefer portable packages or MSIX/AppX installations. The matching parameter is `--installer-type`, which will override the settings.
+
+Allowed values as of version 1.12.470 include: `appx`, `burn`, `exe`, `font`, `inno`, `msi`, `msix`, `msstore`, `nullsoft`, `portable`, `wix`, `zip`
+
+By default, and with all other properties being equal, WinGet defaults to the installer type that is listed first in the manifest's installer YAML if the package has not been installed yet.  If it is already installed, the same installer type will be required to ensure a proper upgrade.
 
 ```json
     "installBehavior": {