Add glm dependency to README

Author: 9prady9

README.md

@@ -16,6 +16,7 @@ You can find the most recent and updated documentation [here](http://arrayfire.o
 - [glbinding](https://github.com/cginternals/glbinding)
 - [GLFW](http://www.glfw.org/) or [SDL2](https://www.libsdl.org/)
 - [freetype](http://www.freetype.org/)
+- [glm](https://glm.g-truc.net) - not a runtime dependency
 - [boost](https://www.boost.org/) - not a runtime dependency.
 - [FreeImage](http://freeimage.sourceforge.net/) (optional).
 - [fontconfig](http://www.freedesktop.org/wiki/Software/fontconfig/) on Linux and OSX.