Add comment about pg_ctl stop

src/tools/pgindent/README

@@ -28,6 +28,8 @@ This can format all PostgreSQL *.c and *.h files, but excludes *.y, and
 6) Do a full test build:
 
 	run configure
+	# stop is only necessary if it's going to install in a location with an
+	# already running server
 	pg_ctl stop
 	gmake -C src install
 	gmake -C contrib install