Improve visibility of plugin details to 'ordinary' users

We appear to have many users who fail to see the README and CHANGELOG info. Others see GitHub pages and get put off.

We discussed it again at the dev meeting in April and amongst other things noted:

We need to a better way to show new plugin changes to users.
Have a plugin folder with notes which contain the changelogs and documentation. The plugin updater would update the notes
API to open an URL to open documentation and use markdown to open a read-only window
Show the markdown of the readme.md which is already shipped in a read-only editor popup.
How can we add something simple to the sidebar to expose more documentation?

This is particularly important when we move to auto-updating for most plugin changes.

Most of the work is for @EduardMe this time.

But it struck me, @mikeerickson, to ask you to update the way npc does releases to include the CHANGELOG and README in the release. Therefore it will be local and can display quickly and without internet connection. I suggest we continue to host images/videos elsewhere, and accept that they are sometimes unavailable.