README: Clarify where to run the generate_git script from

Just running the script from the current directory doesn't give a clue:

    Cant find series file at /series

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/README b/README
index 00ea32b..f6afc7f 100644
--- a/README
+++ b/README
@@ -62,9 +62,10 @@
 
 The tree can be exported as a git tree, based on the kernel.org upstream
 release described in the file KERNEL_VERSION.  To do this run the
-scripts/generate_git script.  For more details on the environment
-variables this script expects and how it works, please see the built in
-help in the script by running:
+scripts/generate_git script from a git repository that already contains
+that version.  For more details on the environment variables this script
+expects and how it works, please see the built in help in the script by
+running:
 	scripts/generate_git --help