Add glew.

extern/glew/README.txt

@@ -0,0 +1,18 @@
+See doc/index.html for more information.
+
+If you downloaded the tarball from the GLEW website, you just need to:
+
+    Unix:
+
+        make
+
+    Windows:
+
+        use the project file in build/vc6/
+
+If you wish to build GLEW from scratch (update the extension data from
+the net or add your own extension information), you need a Unix
+environment (including wget, perl, and GNU make).  The extension data
+is regenerated from the top level source directory with:
+
+        make extensions