is_cool=True from the docs

[gabn88]

If I look into the code I don't see an is_cool model field ( https://github.com/xtrinch/fcm-django/blob/master/docs/index.rst#migration-to-v10
). It would be cool to have it or to explain how to get it. Everybody loves to send hot messages to cool devices to heat them up.

[xtrinch]

This field does not exist and is only used as an example on using a filter in the docs, if you want additional fields on the device model you'll need to override it