Add LogDog project and component documentation.

logdog/server/cmd/logdog_archivist/README.md

Index: logdog/server/cmd/logdog_archivist/README.md
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+LogDog Archivist
+================
+
+The LogDog Archivist is tasked by the **Coordinator**, and has the job of
+collecting log stream data from **Intermediate Storage** (where it was deposited
+by the **Collector**) and loading it into **Archival Storage**. It does this by
+scanning through **Intermediate Storage** for consecutive log entries and
+constructing archive files:
+
+* The Logs file, consisting of Record IO entries containing the
+  `LogStreamDescriptor` protobuf followed by every `LogEntry` protobuf in the
+  stream.
+* The Index file, consisting of a `LogStreamDescriptor` RecordIO entry followed
+  by a `LogIndex` protobuf entry.
+* An optional Data file, consisting of the reassembled contiguous raw log stream
+  data.
+
+These files are written into **Archival Storage** by the **Archivist** during
+archival. After archival is complete, the **Archivist** notifies the
+**Coordinator** and the log stream's state is updated.
+
+**Archivist** microservices are designed to operate cooperatively as part of
+a scaleable cluster. Deploying additional **Archivist** instances will linearly
+increase the archival throughput.
+
+**Archivist** instances load the global LogDog configuration, and are
+additionally configured via the `Archivist` configuration message in
+[config.proto](../../../api/config/svcconfig/config.proto). Configuration
+is loaded from the **Coordinator** and the **Configuration Service**.
+
+## Staging
+
+Archival is initially written to a staging storage location. After the archival
+successfully completes, the staged files are moved to permanent location using
+an inexpensive rename operation.
+
+## Incomplete Logs
+
+It is possible for log streams to be missing data at the time of archival. Each
+archival request includes a completeness threshold. If the archival request is
+younger than that threshold and the archival fails due to error or
+incompleteness, the request will be returned to the queue for future processing.
+
+If, however, the archival request is older than that threshold, a best-effort
+archival where missing logs are not considered errors will be executed. This
+will gracefully skip over any missing log entries, resulting in an incomplete
+log stream.
+
+This threshold is configured using the `archive_settle_delay` option in the
+`Coordinator` configuration message.