Help Wanted: Document flow of adding bot to channel during webhook subscription creation

[mickmister]

Since the App's bot needs to be a member of the channel to create posts, the App needs to use the acting user's token to add the bot during the time of subscription creation

_Issue created from a Mattermost message by @mickmister._

[cwarnermm]

@asatkinson - Do you have resources available to help draft this dev docs update?