Added documentation for windows users attempting to set environment variables (#822)

CONTRIBUTING.md

@@ -5,10 +5,17 @@ If you would like to contribute to spotipy follow these steps:
 ### Export the needed environment variables
 
 ```bash
+# Linux or Mac
 export SPOTIPY_CLIENT_ID=client_id_here
 export SPOTIPY_CLIENT_SECRET=client_secret_here
 export SPOTIPY_CLIENT_USERNAME=client_username_here # This is actually an id not spotify display name
 export SPOTIPY_REDIRECT_URI=http://localhost:8080 # Make url is set in app you created to get your ID and SECRET
+
+# Windows
+$env:SPOTIPY_CLIENT_ID="client_id_here"
+$env:SPOTIPY_CLIENT_SECRET="client_secret_here"
+$env:SPOTIPY_CLIENT_USERNAME="client_username_here" 
+$env:SPOTIPY_REDIRECT_URI="http://localhost:8080" 
 ```
 
 ### Create virtual environment, install dependencies, run tests: