Document the 'maintenance' sub command

[jessereynolds]

As far as I can tell the only documentation of the maintenance subcommand is in the online help.

From 1.6.0:

$ flapjack maintenance --help
NAME
    maintenance - Show, create and delete maintenance windows

SYNOPSIS
    flapjack [global options] maintenance create [--check arg|-c arg] [--duration arg|-d arg] [--entity arg|-e arg] [--reason arg|-r arg] [--start arg|--started arg|--starting arg|-s arg] [--type arg|-t arg]
    flapjack [global options] maintenance delete [--apply arg|-a arg] [--check arg|-c arg] [--duration arg|-d arg] [--entity arg|-e arg] [--finish arg|--finished arg|--finishing arg|--remain arg|--remained arg|--remaining arg|--end arg|-f arg] [--reason arg|-r arg] [--start arg|--started arg|--starting arg|-s arg] [--state arg|--st arg] [--type arg|-t arg]
    flapjack [global options] maintenance show [--check arg|-c arg] [--duration arg|-d arg] [--entity arg|-e arg] [--finish arg|--finished arg|--finishing arg|--remain arg|--remained arg|--remaining arg|--end arg|-f arg] [--reason arg|-r arg] [--start arg|--started arg|--starting arg|-s arg] [--state arg|--st arg] [--type arg|-t arg]

COMMANDS
    show   - Show maintenance windows according to criteria (default: all ongoing maintenance)
    delete - Delete maintenance windows according to criteria (default: all ongoing maintenance)
    create - Create a maintenance window

We should add a page to the Using section on flapjack.io/docs about how to use this most useful command line function with some handy examples such as 'show all maintenances ending in the next 12 hours'