Clarifying README.

Change-Id: I120cae8ff06be2d723d1f982c5424e720b0d6930
diff --git a/tools/tradefed-host/README b/tools/tradefed-host/README
index bd29c23..545d4b4 100644
--- a/tools/tradefed-host/README
+++ b/tools/tradefed-host/README
@@ -38,8 +38,12 @@
 To run a test plan on a single device:
 
 1. Make sure you have at least one device connected
-2. Launch the cts-tradefed console by running the 'cts-tradefed' script at
-android-cts/tools/cts-tradefed
+2. Launch the cts-tradefed console by running the 'cts-tradefed'. If you've
+downloaded and extracted the CTS zip, the script can be found at
+  android-cts/tools/cts-tradefed
+Or else if you are working from the Android source tree and have run make cts,
+the script can be found at
+  out/host/linux-x86/cts/android-cts/tools/cts-tradefed
 3. Type:
 'run cts --plan CTS' to run the default CTS plan