Added extensions to X11 conformance section.

docs/compat.dox

@@ -62,6 +62,23 @@ GLFW uses the
 to provide file drop events.  If the application originating the drag does not
 support this protocol, drag and drop will not work.
 
+GLFW uses the XInput 2 extension to provide sub-pixel cursor motion events.  If
+the running X server does not support this version of this extension, cursor
+motion will be snapped to the pixel grid.
+
+GLFW uses the XRandR 1.3 extension to provide multi-monitor support.  If the
+running X server does not support this version of this extension, multi-monitor
+support will not function and only a single, desktop-spanning monitor will be
+reported.
+
+GLFW uses the XRandR 1.3 and Xf86vidmode extensions to provide gamma ramp
+support.  If the running X server does not support either or both of these
+extensions, gamma ramp support will not function.
+
+GLFW requires the Xkb extension with detectable auto-repeat to provide keyboard
+input.  If the running X server does not support this extension and detectable
+auto-repeat, `glfwInit` will fail.
+
 @section compat_glx GLX extensions
 
 The GLX API is the default API used to create OpenGL contexts on Unix-like