Add README.md link to https://github.com/lifepillar/vim-solarized8 for True Color

[jeffvandyke]

I was going to make an issue for this project to support True Color on terminals that support it, but in lieu of that, a link to https://github.com/lifepillar/vim-solarized8 for folks (like me) looking for such a (neo)vim plugin would be most useful.

IMO, this project's options for easy true solarized color are sub-optimal, the only options being either using the 256 color fallback, which is close, but inaccurate, missing the blue-teal look, or alternatively, a solarized terminal 16-color palette, which is a terrible palette for terminal apps wanting to use colors like Green and DarkGreen with DarkGreen not being even a little bit green, as well as issues like mochajs/mocha#802 showing how text set to a DarkGrey is invisible against 16-color terminal's background. All this is why I want something that works with True Color, but only with vim, which can properly support it.