Bug 1686777 - Add a little documentation to swgl. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D101792

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/a9764bdb94e4dab679f5fff44438b0eb7192bf86

swgl/README.md

@@ -2,3 +2,18 @@ swgl
 ========
 
 Software OpenGL implementation for WebRender
+
+Overview
+--------
+This is a relatively simple single threaded software rasterizer designed
+for use by WebRender. It will shade one quad at a time using a 4xf32 vector
+with one vertex per lane. It rasterizes quads usings spans and shades that
+span 4 pixels at a time.
+
+Building
+--------
+clang-cl is required to build on Windows. This can be done by installing
+the llvm binaries from https://releases.llvm.org/ and adding the installation
+to the path with something like `set PATH=%PATH%;C:\Program Files\LLVM\bin`.
+Then `set CC=clang-cl` and `set CXX=clang-cl`. That should be sufficient
+for `cc-rs` to use `clang-cl` instead of `cl`.