Add basic architecture doc

Author: belak

src/SUMMARY.md

@@ -1,6 +1,7 @@
 # Summary
 
 [Introduction](README.md)
+[Architecture](architecture.md)
 
 # Administrator Guide
 

src/architecture.md

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+# Architecture
+
+At its core, Seabird is an RPC API.
+
+Currently, seabird exports 2 apis: a chat ingestion API, and a plugin API. In
+general, all components only interact with one of these.
+
+## seabird-core
+
+### Chat Ingestion API
+
+The chat ingestion API consists of a single endpoint `IngestEvents` which
+starts a bidirectional stream. Outgoing messages are "commands" which allow
+seabird to inform the ingestion process to perform some sort of action, like
+sending a message or joining a channel. Incoming messages are the ingested
+events, allowing users of this API to inform seabird of new messages or other events.
+
+### Plugin API
+
+The plugin API consists of both an event stream and a number of methods which
+can be used to cause things to happen.
+
+The streaming endpoint `StreamEvents` is a firehose of all events occuring
+across all seabird chat backends.
+
+For callable methods, the `SendMessage` endpoint is the most commonly used.
+
+## Chat Backends
+
+Chat backends interact with the Chat Ingestion API and are used to connect some
+sort of chat server or network into the Seabird ecosystem.
+
+## Plugins
+
+Plugins interact with the Plugin API and are used to provide some sort of
+additional functionality, similar to what a chat bot would do.