Improve readme

README.md

@@ -1,9 +1,16 @@
-
+An OpenGL renderer for [Mapbox vector tiles](https://www.mapbox.com/blog/vector-tiles),
+implemented in C++, targeting iOS & OSX.
 
 # Build instructions
 
 ## Mac OS X
 
+You may need to install [glfw3](http://www.glfw.org/docs/latest/):
+
+```
+brew install homebrew/versions/glfw3
+```
+
 ```
 mkdir xcode
 cd xcode
@@ -13,7 +20,6 @@ cmake .. -G Xcode
 Then open the xcode project and build.
 
 
-
 # Style protobuf
 
 We're encoding the styling information as a protocol buffer, according to