doc tweak
diff --git a/README.md b/README.md
index a493b0d..cdb1482 100644
--- a/README.md
+++ b/README.md
@@ -186,7 +186,9 @@
         erl_build.sh
 
 * the output is logged into `$ERL_TOP/build_*.txt` if required
-* at this point I usually duplicate the OTP source tree for later
+* at this point I usually duplicate the OTP source tree for later, as
+Running the same command swapping the parameters will clean out
+the build tree completely.
 
         robocopy $ERL_TOP /relax/release/$OTP_REL -mir