docs(install): Add winget install instruction

The 'difftastic' is not linked like in the other sections because winget does not have a package-frontend website. We could link to the package source [pkgsrc], but that seems inappropriate for end-users.

[pkgsrc]: https://github.com/microsoft/winget-pkgs/tree/master/manifests/w/Wilfred/difftastic

manual/src/installation.md

@@ -52,6 +52,12 @@ $ sudo pkg install difftastic
 
 ### Windows
 
+If you're a Windows user using **Windows Package Manager** (*WinGet*), you can install difftastic with `winget`.
+
+```
+$ winget install difftastic
+```
+
 If you're a Windows user using **Scoop**, you can install
 [difftastic](https://scoop.sh/#/apps?q=difftastic)
 with `scoop`.