book: Add architecture section.

book/src/SUMMARY.md

@@ -4,6 +4,8 @@
 - [Ideology](IDEOLOGY.md)
 - [Development](development.md)
 - [Tutorial](tutorial.md)
+- [Architecture](architecture/architecture.md)
+  - [Node design](architecture/nodes.md)
 - [Client](clients/clients.md)
   - [darkfid JSON-RPC API](clients/darkfid_jsonrpc.md)
   - [cashierd JSON-RPC API](clients/cashierd_jsonrpc.md)

book/src/architecture/architecture.md

@@ -0,0 +1,5 @@
+Architecture design
+===================
+
+This section of the book shows the software architecture of DarkFi and
+the network implementations.

book/src/architecture/nodes.md

@@ -0,0 +1,5 @@
+DarkFi nodes
+============
+
+The core daemon used for wallet management, transaction building, and
+general operations is called `darkfid`.