More design details

doc/DESIGN.md

@@ -26,6 +26,9 @@ The diagram below shows a high-level view of a rqlite node.
                  ┌───────────────────────────────────────────────┐
                  │                   sqlite3.c                   │
                  └───────────────────────────────────────────────┘
+                 ┌───────────────────────────────────────────────┐
+                 │                 RAM or disk                   │
+                 └───────────────────────────────────────────────┘
 
 ## Log Compaction
 rqlite automatically performs log compaction. After a configurable number of changes rqlite snapshots the SQLite database, and truncates the Raft log. This is a technical feature of the Raft consensus system, and most users of rqlite need not be concerned with this.