Added caveat for context sharing on Windows.

WGL prohibits sharing with contexts current on non-main threads.

Fixes #402.

include/GLFW/glfw3.h

@@ -1631,6 +1631,9 @@ GLFWAPI void glfwWindowHint(int target, int hint);
  *  `GLFW_ICON,` it will be set as the icon for the window.  If no such icon is
  *  present, the `IDI_WINLOGO` icon will be used instead.
  *
+ *  @remarks __Windows:__ The context to share resources with may not be current
+ *  on any other thread.
+ *
  *  @remarks __OS X:__ The GLFW window has no icon, as it is not a document
  *  window, but the dock icon will be the same as the application bundle's icon.
  *  For more information on bundles, see the