Add note on extra-cmake-modules dependency

The Wayland backend requires this package to generate headers from XML
protocol descriptions.  Although this package was created by the KDE
project, it does not depend on any part of KDE.

Fixes #1013.

[ci skip]

README.md

@@ -89,6 +89,9 @@ in the documentation for more information.
 
 GLFW itself depends only on the headers and libraries for your window system.
 
+The (experimental) Wayland backend also depends on the `extra-cmake-modules`
+package, which is used to generated Wayland protocol headers.
+
 The examples and test programs depend on a number of tiny libraries.  These are
 located in the `deps/` directory.