add samples/android/common directory

Change-Id: I6d97cd7bf6bde02cf696ab97e7987bada75a3238
diff --git a/common/readme.txt b/common/readme.txt
new file mode 100644
index 0000000..4c37f64
--- /dev/null
+++ b/common/readme.txt
@@ -0,0 +1,3 @@
+samples/android/common: use this directory for code that will be included in more than one sample.
+
+Recommended development technique is to ln -s common into your sample's src directory.