Add FAQ (#485)

* Add FAQ

* Add FAQ to README

FAQ.md

@@ -0,0 +1,31 @@
+## Frequently Asked Questions
+
+### Is there a way to get this field?
+
+spotipy can only return fields documented on the Spotify web API https://developer.spotify.com/documentation/web-api/reference/
+
+### Incorrect user
+
+Error:
+
+ - You get `You cannot create a playlist for another user`
+ - You get `You cannot remove tracks from a playlist you don't own`
+
+Solution:
+
+ - Verify that you are signed in with the correct account on https://spotify.com
+ - Remove your current token: `rm .cache-{userid}`
+ - Request a new token by adding `show_dialog=True` to `util.prompt_for_user_token(username,scope=scope,show_dialog=True)`
+ - Check that `spotipy.me()` shows the correct user id
+
+### 401 Unauthorized
+
+Error:
+
+    spotipy.exceptions.SpotifyException: http status: 401, code:-1 - https://api.spotify.com/v1/
+    Unauthorized.
+
+Solution:
+
+ - You are likely missing a scope when requesting the endpoint, check
+https://developer.spotify.com/web-api/using-scopes/