add nerd font install help

src/powershell-extended/ENHANCEMENTS.md

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+# How to Enhance Your PowerShell Terminal Experience in Dev Containers
+
+Your Dev Container (or Codespace) uses the [`powershell-extended`](https://github.com/jpawlowski/devcontainer-features/tree/main/src/powershell-extended)
+feature which provides a great out-of-box experience for your PowerShell.
+
+Some advanced visual features depend on the availability of a Nerd Font, which are fonts that are extended with special characters
+to display glyphs/icons and other graphics right in your terminal window.
+
+By installing such font and tweaking some settings of your Dev Container and Visual Studio Code settings, you unlock the
+best experience for your PowerShell terminal.
+
+## Install Nerd Font
+
+Since the VSCode default font (on Windows) is _Cascadia Mono_, we recommend to install an updated version of that very
+same font directly from [Github/Microsoft/Cascadia-Code](https://github.com/microsoft/cascadia-code/releases).
+Since its [May 2024 release](https://github.com/microsoft/cascadia-code/releases/tag/v2404.23) it provides a native
+Nerd Font variant with all the nice details we care about. Since it is not bundles with any of the
+Microsoft applications (yet), you will need to install it separately.
+
+However, you may also select any other font from [nerdfonts.com](https://www.nerdfonts.com/).
+
+To make the setup easier for you, you may copy the following lines into your **local** PowerShell
+terminal (that is, a local Windows Terminal window, or a local VSCode window that is not running your Dev Container):
+
+```powershell
+$tempFile = New-TemporaryFile
+$scriptUrl = 'https://raw.githubusercontent.com/jpawlowski/devcontainer-features/main/src/powershell-extended/Install-NerdFont.ps1'
+try {
+    Invoke-WebRequest -Uri $scriptUrl -OutFile $tempFile.FullName -ErrorAction Stop
+    Unblock-File -Path $tempFile.FullName
+    & $tempFile.FullName
+} catch {
+    Write-Error "$_"
+} finally {
+    Remove-Item $tempFile.FullName -Force -ErrorAction Ignore
+}
+```
+
+This will provide an interactive menu for you to select the desired font.
+The install script is designed to be multi-platform and will install fonts on Windows, macOS, and Linux devices.