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>
diff --git a/README b/README
index ed7d270..06f5ef2 100644
--- a/README
+++ b/README
@@ -54,6 +54,11 @@
 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:
 ==================