Add use of glad to tutorial

docs/quick.dox

@@ -45,6 +45,17 @@ In other words:
 - If you _do_ need to include such headers, do it _before_ including the
   GLFW one and it will detect this
 
+If you are using an [extension loader library](@ref context_glext_auto) to
+access modern OpenGL then include the header for that library _before_ the GLFW
+header.  This lets it replace the OpenGL header included by GLFW without
+conflicts.  The following example uses [glad](https://github.com/Dav1dde/glad),
+but the same rule applies to all.
+
+@code
+#include <glad/glad.h>
+#include <GLFW/glfw3.h>
+@endcode
+
 
 @subsection quick_init_term Initializing and terminating GLFW
 
@@ -144,6 +155,15 @@ glfwMakeContextCurrent(window);
 The context will remain current until you make another context current or until
 the window owning the current context is destroyed.
 
+If you are using an [extension loader library](@ref context_glext_auto) to
+access modern OpenGL then this is when to initialize it.  The loader needs the
+context to be current before it can load from it.  The following example uses
+[glad](https://github.com/Dav1dde/glad), but the same rule applies to all. 
+
+@code
+gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
+@endcode
+
 
 @subsection quick_window_close Checking the window close flag