Add reboot command to readme

[will-v-pi]

Add a description of the picotool reboot command to the readme

[lurch]

Probably also makes sense to update the "checking script" so that this (or other similar commands) can't get overlooked in future? 🤔

[will-v-pi]

Probably also makes sense to update the "checking script" so that this (or other similar commands) can't get overlooked in future? 🤔

The reboot command would need it's own more complex handling added to the script, as the current script checks for the array of all the commands, but the reboot command is created outside of that array

Given it's unlikely there'll be another command like reboot which needs this handling, as I've moved to using load_guts, info_guts etc instead of calling reboot_cmd->execute, I think it's fine to not add handling to the script - now the command is in the README, it'll still be updated if the help text changes anyway

[lurch]

now the command is in the README, it'll still be updated if the help text changes anyway

Fair enough 👍

[lurch]

@kilograham Could we get a brief explanation of what the diagnostic partition is please?