document how to find non-shared string dups

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt
index 079bac3..01c0d3c 100644
--- a/docs/keep_data_small.txt
+++ b/docs/keep_data_small.txt
@@ -145,6 +145,11 @@
 If you have less than ~300 bytes of global data - don't bother.
 
 
+		Finding non-shared duplicated strings
+
+strings busybox | sort | uniq -c | sort -nr
+
+
 		gcc's data alignment problem
 
 The following attribute added in vi.c: