Add LogDog project and component documentation.

logdog/appengine/cmd/coordinator/README.md

+LogDog Coordinator
+==================
+
+The LogDog Coordinator is an AppEngine application that serves as a central
+coordinating and interactive authority for a LogDog instance. The Coordinator
+is responsible for:
+
+* Supplying logs to end users through its logs API.
+* Coordinating log stream state throughout its lifecycle.
+* Handling Butler Prefix registration.
+* Acting as a configuration authority for its deployment.
+* Accepting stream registrations from **Collector** instances.
+* Dispatching archival tasks to **Archivist** instances.
+
+## Modules
+
+A **Coordinator** occupies the AppEngine space of a given cloud project, and
+assumes ownership of that project's resources. It is composed of several
+cooperative AppEngine modules.
+
+### Default
+
+The [default](vmuser/) module exposes the
+[Logs API](../../../api/endpoints/coordinator/logs/v1/) for log stream querying
+and consumption.
+
+It is currently an AppEngine Managed VM, since Managed VMs are the only type of
+AppEngine instance that can use gRPC, and gRPC is needed to read from BigTable.
+
+### Services
+
+The [services](services/) module exposes management endpoints to the instance's
+microservices, notably the [Collector](../../../server/cmd/logdog_collector) and
+[Archivist](../../../server/cmd/logdog_archivist) microservices. These endpoints
+are used to coordinate the microservice-managed aspects of the log stream
+lifecycle.
+
+### Backend
+
+The [backend](backend/) module hosts the [Tumble](/tumble) journaled processing
+service, which handles log stream lifecycle transitions and deadlines.
+
+### Static
+
+The [static](static/) module hosts static content, including:
+* The LogDog Web Application
+* The LogDog Lightweight Stream Viewer
+* `rpcexplorer`