README: Explain how to add a new API function

This is not obvious so add a little note about it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Message-Id: <20211107224346.3181320-2-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

README

@@ -77,6 +77,15 @@ More work remains to support all of libfdt, including access to numeric
 values.
 
 
+Adding a new function to libfdt.h
+---------------------------------
+
+The shared library uses libfdt/version.lds to list the exported functions, so
+add your new function there. Check that your function works with pylibfdt. If
+it cannot be supported, put the declaration in libfdt.h behind #ifndef SWIG so
+that swig ignores it.
+
+
 Tests
 -----