Update readme to point to Khronos landing page.

README.md

+Also see the Khronos landing page for glslang as a reference front end:
+
+https://www.khronos.org/opengles/sdk/tools/Reference-Compiler/
+
+The above page includes where to get binaries, and is kept up to date
+regarding the feature level of glslang.
+
 glslang
 =======