Add stubs for Markdown documentation

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127560220

docs/app-engine-architecture.md

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+# App Engine architecture
+
+This document contains information on the overall architecture of the Domain Registry project as it is implemented in App Engine.
+
+## Modules
+
+## Task queues
+
+## Cron tasks
+
+## Datastore entities
+
+## Cloud Storage buckets

docs/code-structure.md

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+# Code structure
+
+An overall look at the structure of the Domain Registry code.

docs/configuration.md

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+# Configuration
+
+Information on how to configure/customize an installation.

docs/developing.md

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+# Developing
+
+Advice on how to do development on the Domain Registry codebase (including how to set up an IDE environment and run tests).

docs/extension-points.md

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+# Extension points
+
+The various places the system can be extended by plugging in additional code.

docs/install.md

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+# Installation
+
+Information on how to download and install the Domain Registry project and get a working running instance.

docs/registry-tool.md

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+# Registry tool
+
+Information on registry_tool commands.