add architectural info to readme

README.md

@@ -1,3 +1,34 @@
 # 开颜
 
 Kaiyan is an analytics platform for 7TV emotes on Twitch.
+It provides various indicators of emote popularity and allows identifying the least used emotes so that streamers can quickly decide which ones to remove when it's time for a cleanup.
+
+## Architecture
+
+Kaiyan is built as a collection of small components.
+
+### Subscription manager
+
+The subscription manager registers new channels and their 7TV emotes.
+It communicates to the indexer to describe emotes which are added or removed in real time.
+It also handles channels that revoke their subscription.
+
+This is the component which handles OAuth2 authentication with Twitch.
+
+### Message ingest
+
+Message ingest consumes chat messages, parsing metadata and content.
+It emplaces an event for each message onto Kafka for the indexers to process.
+
+### Indexer
+
+The indexer records emote usage in ClickHouse, on open-source column store database for online analytical processing.
+
+### API
+
+The API computes full metrics on demand from what the indexer has recorded.
+
+### Load tester
+
+The load tester is an independent component that pushes a high volume of simulated messages into message ingest.
+This produces internal metrics to determine how message ingest, indexers, Kafka, and ClickHouse can scale relative to each other.