Add a link to "So you want to write a GUI framework"

[hammerandtongs]

https://www.cmyr.net/blog/gui-framework-ingredients.html

Lays out a pretty broad but necessary set of items for a full featured gui framework.

Developers evaluating whether Rust is areweguiyet would do well to read this before they begin looking at Rust frameworks.

As a feature request for areweguiyet, what about a feature matrix based around this list?

[Jay]

Disposition to merge

[Mads Marquart]

Thanks for the link, I've put it on the site in #117.

I've moved the feature matrix suggestion to #21, since that's where we're discussing reworking tags/categories.