Make common/README.txt more succinct

This will get copied into other samples, and I don't want to
scare users away from the sample they're currently browsing.

Change-Id: I6f86f29e4fd73abb44ada9df848a4e529df97527
diff --git a/common/README.txt b/common/README.txt
index 1f28b00..1320cfc 100644
--- a/common/README.txt
+++ b/common/README.txt
@@ -1,4 +1 @@
 This directory contains shared classes referenced by some samples.
-If you're exploring the samples, we recommend you start in the other
-directories first.