docs: Add automatic Windows support to Readme

Author: marisa

README.md

@@ -31,6 +31,16 @@ curl -L https://git.io/rustlings | bash -s mypath/
 
 This will install Rustlings and give you access to the `rustlings` command. Run it to get started!
 
+## Windows
+
+You can run:
+
+```ps
+Invoke-WebRequest https://git.io/rustlings-win | Select-Object -ExpandProperty Content | Out-File $env:TMP/install_rustlings.ps1; Unblock-File $env:TMP/install_rustlings.ps1; Invoke-Expression $env:TMP/install_rustlings.ps1
+```
+
+To install Rustlings. Same as on MacOS/Linux, you will have access to the `rustlings` command after it.
+
 ## Manually
 
 Basically: Clone the repository, checkout to the latest tag, run `cargo install`.