README: Explain how to generate the CHANGELOG file.

Since the top-level README file refers the reader to the CHANGELOG,
it's worth mentioning how to generate it.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>

README

@@ -54,6 +54,11 @@ In case of problems see the CHANGELOG and CREDITS files to find out
 who contributed the specific port. The MAINTAINERS file lists board
 maintainers.
 
+Note: There is no CHANGELOG file in the actual U-Boot source tree;
+it can be created dynamically from the Git log using:
+
+	make CHANGELOG
+
 
 Where to get help:
 ==================