Clarify library dependencies

README.md

@@ -45,7 +45,12 @@ This will help both us and other people experiencing the same bug.
 
 ## Dependencies
 
-GLFW bundles a number of dependencies in the `deps/` directory.
+GLFW itself needs only the headers and libraries for your window system.  It
+does not need the headers for any context creation API (WGL, GLX, EGL, NSGL) or
+client API (OpenGL, OpenGL ES) to enable support for them.
+
+GLFW bundles a number of dependencies in the `deps/` directory.  These are only
+used by the tests and examples and are not required to build the library.
 
  - [getopt\_port](https://github.com/kimgr/getopt_port/) for examples
    with command-line options