Add portable mode docs to release branch 1.18 (#655)

* added first draft of docs

* changed headings and subheadings

* Reword, add an Upgrading section

---------

Co-authored-by: Dustin Howett <duhowett@microsoft.com>

TerminalDocs/portable-mode.md

@@ -0,0 +1,58 @@
+---
+title: Windows Terminal Portable Mode
+description: Learn how to use Portable mode on Windows Terminal.
+author: nguyen-dows
+ms.author: chrnguyen
+ms.date: 04/10/2023
+ms.topic: how-to 
+---
+
+# Windows Terminal Portable
+
+As of version 1.17, Windows Terminal supports being deployed in ["Portable mode"]. Portable mode ensures that all data
+created and maintained by Windows Terminal is saved next to the application so that it can be more easily moved across
+different environments.
+
+Portable mode is supported by the [unpackaged "ZIP" distribution]. This is an officially-supported mode of execution
+where Windows Terminal stores its settings in a `settings` folder next to `WindowsTerminal.exe`.
+
+Portable mode is not supported in the packaged ("MSIX bundle") distribution of Windows Terminal.
+
+## Why use Portable mode? 
+
+The unpackaged and portable mode distribution of Windows Terminal allows you to use Terminal without installing it
+globally, e.g. on systems where you may not have permission to install MSIX packages or download software from the
+Microsoft Store.
+
+Portable mode allows you to carry around (and archive!) a preconfigured installation of Windows Terminal and run it from
+a network share, cloud drive or USB flash drive. Any such installation is self-contained and will not interfere with
+other installed distributions of Windows Terminal.
+
+## Enabling Portable mode
+
+Portable mode needs to be enabled manually. After unzipping the Windows Terminal download, create a file named `.portable` next to `WindowsTerminal.exe`.
+
+> [!NOTE]
+> Windows Terminal will not automatically reload its settings when you create the portable mode marker file.
+> This change will only apply after you relaunch Terminal.
+
+Windows Terminal will automatically create a directory named `settings` in which it will store both settings and runtime
+state such as window layouts.
+
+![Windows Terminal portable mode](./images/portable-mode.png)
+
+## Disabling Portable mode
+
+You can restore Portable mode unpackaged installation to its original configuration, where settings are stored in
+`%LOCALAPPDATA%\Microsoft\Windows Terminal`, by removing the `.portable` marker file from the directory containing
+`WindowsTerminal.exe`.
+
+If you wish to reenable portable mode, you can create a new `.portable` marker file next to `WindowsTerminal.exe`.
+
+## Upgrading a Portable mode Install
+
+You can upgrade a portable mode installation of Windows Terminal by moving the `.portable` marker file and the
+`settings` directory to a newly-extracted unpackaged version of Windows Terminal.
+
+["Portable mode"]: https://en.wikipedia.org/wiki/Portable_application
+[unpackaged "ZIP" distribution]: https://github.com/microsoft/terminal/releases