Add an overview section to README

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 README for lorry-controller
 ===========================
 
-Lorry Controller mirrors Troves and mirrors or converts upstream
-projects into git repositories on the local Trove. Lorry Controller
-reads a configuration file (see below) and runs the Lorry program
-against the intended targets at suitable intervals.
+Overview
+--------
+
+Lorry Controller, or LC for short, manages the importing of source
+code from external sources into git repositories on a Trove. A Trove
+is a component in the Baserock system for hosting source code, and LC
+runs inside a Trove.
+
+LC uses the Lorry tool to do the actual import. Lorry can read code
+from several different version control systems, and convert them to
+git. External repositories can be specfied individually, as Lorry
+`.lorry` specification files. In addition, LC can be told to mirror
+all the git repositories on another Trove.
+
+LC runs Lorry for the right external repositories, and takes care of
+running a suitable number of Lorry instances concurrently, and
+recovering from any problems. LC has a web based administration
+interface, and an HTTP API for reporting and controlling its state.
+
+This README file documents the LC configuration file and general use.
+For the architecture of LC and the HTTP API, see the `ARCH` file.
 
 Lorry Controller configuration
 ------------------------------