Add informative samples README.md

Also updates stale documentation with informative links.
diff --git a/samples/adsensehost/README.md b/samples/adsensehost/README.md
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+# Where are the samples?
+
+The AdSense Host API samples are now hosted separately from the client library
+project. You can find them
+[here](https://github.com/googleads/googleads-adsensehost-examples).