Add README to handlers folder

src/handlers/README.md

@@ -0,0 +1,10 @@
+This folder contains all of the RPC handlers.
+
+Generally, handlers parse the request and call the appropriate method of `BackendInterface`
+to read from the database.
+
+Certain RPCs, such as `fee` and `submit`, are just forwarded to `rippled`, and the response
+is propagated back to the client.
+
+If the database returns a timeout, an error is returned to the client.
+This automatically happens, and is caught at a higher level, outside of the handler.