Add documentation on poll messages and an outline of Code structure

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133713736

docs/code-structure.md

@@ -1,3 +1,58 @@
 # Code structure
 
-An overall look at the structure of the Domain Registry code.
+This document contains information on the overall structure of the code, and how
+particularly important pieces of the system are implemented.
+
+## Dagger dependency injection
+
+## Bazel build system
+
+## Flows
+
+## Commit logs and backups
+
+## Cursors
+
+## Mapreduces
+
+## Actions and servlets
+
+## Foreign key indexes
+
+## Point-in-time accuracy
+
+## Guava
+
+## EPP resource hierarchy
+
+## Poll messages
+
+Poll messages are the mechanism by which EPP handles asynchronous communication
+between the registry and registrars. Refer to
+[RFC 5730 Section 2.9.2.3](https://tools.ietf.org/html/rfc5730#section-2.9.2.3)
+for their protocol specification.
+
+Poll messages are stored by the system as entities in Datastore. All poll
+messages have an event time at which they become active; any poll request before
+that time will not return the poll message. For example, every domain when
+created speculatively enqueues a poll message for the automatic renewal of the
+domain a year later. This poll message won't be delivered until that year
+elapses, and if some change to the domain occurs prior to that point, such as it
+being deleted, then the speculative poll message will be deleted and thus never
+delivered. Other poll messages are effective immediately, e.g. the poll message
+generated for the owning registrar when another registrar requests the transfer
+of a domain. These messages are written out with an event time of when they were
+created, and will thus be delivered whenever the registrar next polls for
+messages.
+
+`PollMessage` is the abstract base class for the two different types of poll
+messages that extend it:
+
+*   **`Autorenew`** - A poll message corresponding to an automatic renewal of
+    a domain. It recurs annually.
+*   **`OneTime`** - A one-time poll message used for everything else.
+
+Queries for poll messages by the registrar are handled in `PollRequestFlow`, and
+poll messages are ACKed (and thus deleted) in `PollAckFlow`.
+
+## Security