Bug 1587650 - Update README.md with more informative intro r=gw

Addresses issues with the README in [mozilla bug 1587650](https://bugzilla.mozilla.org/show_bug.cgi?id=1587650).
[import_pr] From servo#3772

Differential Revision: https://phabricator.services.mozilla.com/D49501

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/fc99c73779480bdc4f82b2cb6e91530cddd48f40

README.md

@@ -2,7 +2,7 @@
 
 [![Version](https://img.shields.io/crates/v/webrender.svg)](https://crates.io/crates/webrender)
 
-GPU renderer for the Web content, used by Servo.
+WebRender is a GPU-based 2D rendering engine written in [Rust](https://www.rust-lang.org/). [Firefox](https://www.mozilla.org/firefox), the research web browser [Servo](https://github.com/servo/servo), and other GUI frameworks draw with it. It currently uses the OpenGL API internally.
 
 Note that the canonical home for this code is in gfx/wr folder of the
 mozilla-central repository at https://hg.mozilla.org/mozilla-central. The