Create README.

Author: Doug Sigelbaum

README.md

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+# Background
+New snippets on developer.android.com/guides will be dynamically linked to this repo instead of 
+written as embedded HTML on the page.
+
+# Contributing
+We accept your pull requests! The code must compile and show best practices. Runtime verification 
+is not needed; code irrelevant to the doc can be stubbed out.