Add example for virtualenv integration in cli docs (#2234)

docs/cli.rst

@@ -56,6 +56,18 @@ If you are constantly working with a virtualenv you can also put the
 bottom of the file.  That way every time you activate your virtualenv you
 automatically also activate the correct application name.
 
+Edit the activate script for the shell you use. For example:
+
+Unix Bash: ``venv/bin/activate``::
+
+    FLASK_APP=hello
+    export FLASK_APP
+
+Windows CMD.exe: ``venv\Scripts\activate.bat``::
+
+    set "FLASK_APP=hello"
+    :END
+
 Debug Flag
 ----------