Developer Guide broadly edited to resemble current project status

[meldra]

This is a fix for #129 but it is also more than that because this is a page that's had chunks added and removed over a decade, and reads like an excerpt from elsewhere to begin with. So:

• The rolecode was wrong... but also...
• The appendix wasn't referenced from anywhere.
• Another appendix was referenced, but does not exist on the page.
• There are mentions of "the example codemeta document" but no indication of what or where that is
• There is general rewording of some sections to make more sense.
• There was also a lot of future tense, and finally
• No linewrapping, so lines were way too wide to be read comfortably and future diffs would still be very chunky.