README: Explain why cd is necessary with run_dev.sh

Change-Id: I592e22f190f319b370b8a99cb7a359856f0fcbf2

README

diff --git a/README b/README
index dfc8497..4b07e4b 100644
--- a/README
+++ b/README
@@ -22,11 +22,12 @@
 
 TESTING
 
-  cd /path/to/repositories
+  cd /path/to/repositories  # Don't run from the gitiles repo.
   /path/to/gitiles/tools/run_dev.sh
 
 This will recompile and start a development server.
-Open http://localhost:8080/ to view your local copy of gitiles.
+Open http://localhost:8080/ to view your local copy of gitiles, which will serve
+any repositories under /path/to/repositories.
 
 
 ECLIPSE