Add contributing guide, screenshots are on website
Improve wording in readme

It's not published on GitHub any more.
Update links to the maintained version of RNB

The gist is no longer maintained.

This repo however will continue to use my simplified version, because
I didn't want to spend time to maintain support for
8 or 16 bit colour terminals with this colorscheme.
Separate Todo from DiffAdd; remove obsolete

Also remove the custom HighlightedyankRegion; uneeded any more because
neovim has a highlighted yank feature built in where you can choose the
highlight to use (I use DiffAdd currently).
fix warningmsg not displaying correctly

for some reason it was set incorrectly and the link wasn't being
mute fold lines more

- highlight strings
- fix warningmsg hi
add screenshots
bold titles to stand out
colour tweaking
initial commit