PluginDownloader: improve README.md

featuring ~~faked~~ log of installing Wikipedia plugin.

[CI SKIP]
Merge pull request #950 from Mikaela/plugindownloader

PluginDownloader: improve README.md
Aka: Fix typo and help for 'show'

Help for 'aka show' should now mentions the --channel argument (this
was implemented but not mentioned).
Aka: add 'list' command (Closes ProgVal/Limnoria#572)
Aka: update listCommands override to only include built in cmds

The old behavior is superseded by 'aka list'. This makes it easier for people to find Aka plugin's core commands, as
'list Aka' won't be filled with random, irrelevant alias definitions.
Aka: update tests
Aka: catch with error if 'aka list' response is empty
QuoteGrabs: Prevent quote grabs from queries.
Merge pull request #951 from GLolol/aka-1

Aka: Fix typo and help for 'show'
Aka: update list command
Merge branch 'testing', remote-tracking branch 'origin/aka-2' into lilybot