Update README.md to v0.15, fix broken link (#1213)
Update README.md to v0.15, fix broken link (#1213)

* Update build dependency information from "gfx = 0.14" to "gfx = 0.15"
* Fixes "Link to arcsynthesis on OpenGL state machine is dead" (#1213)
** Since the old link is dead and I can't find it in the Wayback Machine Archives my guess is just based on the old URL (gltut[orial]/Basics/Intro What is OpenGL)
gfx-rs/gfx-rs,ColinKinloch/gfx,davll/gfx,JohnColanduoni/gfx,ColinKinloch/gfx,JohnColanduoni/gfx