Update README.md plugin format

[gberche-orange]

• Detail valid shortcut values, inspired from

  k9s/change_logs/release_0.8.0.md

  Line 51 in a07243d

  # The menu mnemonic to trigger the command. Valid values are [a-z], Shift-[A-Z], Ctrl-[A-Z] or Alt-[A-Z]

  excluding Alt removed in

  k9s/change_logs/release_0.8.2.md

  Line 17 in 9066739

  In this quick drop, we've opted to nuke any menu shortcut using the infamous `Alt` key. This includes the new pod kill command that is now `Ctrl-K` and for the most part the column sorting shortcuts for CPU% and MEMORY%. My apologizes to all on this fiasco as it turns out I had remapped opt->alt on my local dev machine and space it while trying to offer different key mappings. Will revisit this in the future when things seemer down a bit. Thank you to all that reported on this!

• Detail dangerous option introduced in K9s/release v0.32.4 #2637 and Ability to mark a plugin as Dangerous/destructive #2604
• Detail OverwriteOutput introduced in Allow overwriting plugin output with command's stdout #2644

[derailed]

@gberche-orange Great catch! Thank you Guillaume.