Added documentation for PowerShell environment variables

docs/quickstart.rst

@@ -50,7 +50,14 @@ to tell your terminal the application to work with by exporting the
     $ flask run
      * Running on http://127.0.0.1:5000/
 
-If you are on Windows you need to use ``set`` instead of ``export``.
+If you are on Windows, the environment variable syntax depends on command line
+interpreter. On Command Prompt::
+
+    C:\path\to\app>set FLASK_APP=hello.py
+
+And on PowerShell::
+
+    PS C:\path\to\app> $env:FLASK_APP = "hello.py"
 
 Alternatively you can use :command:`python -m flask`::