Update Unity.gitignore (#3297)

Unity 2020 introduces a new "UserSettings" top-level project folder.
See https://forum.unity.com/threads/whats-the-usersettings-directory.754436/ for more information.

Unity.gitignore

@@ -8,6 +8,7 @@
 /[Bb]uild/
 /[Bb]uilds/
 /[Ll]ogs/
+/[Uu]ser[Ss]ettings/
 
 # MemoryCaptures can get excessive in size.
 # They also could contain extremely sensitive data