Document toggling Powerwall Storm Watch mode

[mikecardwell]

I MITM'd the Tesla app using a custom CA that I loaded onto my iPhone and saw that this is the format of the request that was sent. I then tested it out on my Powerwall 2 by updating the golang lib I'm using to interact with the Tesla API. I have a separate open PR for that here: https://github.com/bogosj/tesla/pull/140

I am likely to make more PR's for other undocumented Powerwall functionality if this PR is accepted.

[purcell-lab]

Also documented here: https://developer.tesla.com/docs/fleet-api#storm_mode

[timdorr]

Thanks!