Add README for developers/samples/android

Change-Id: I44cec1a0c11ecad8169cb9c54c1d9b9ec919dfa3
diff --git a/README.txt b/README.txt
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+This directory contains samples that are distributed with the Android SDK.
+
+New top level directories (categories) are not automatically published. If
+you are adding a new category, you will need to add it to
+develpment/build/sdk.atree.
+
+Note: In the future, the samples will be built using a template system, which
+will automatically import the "common" directory as needed. Until then, if
+you need access to the "common" directory, you will need to have it copied
+during the SDK build process by modifying development/build/sdk.atree.