run yesod, and launch webapp on startup

doc/design/assistant/webapp.mdwn

@@ -2,7 +2,7 @@ The webapp is a web server that displays a shiny interface.
 
 ## security
 
-* Listen only to localhost.
+* Listen only to localhost. **done**
 * Instruct the user's web browser to open an url that contains a secret
   token. This guards against other users on the same system.
 * I would like to avoid passwords or other authentication methods,

doc/git-annex.mdwn

@@ -188,6 +188,12 @@ subdirectories).
 * assistant
 
   Like watch, but also automatically syncs changes to other remotes.
+  Typically started at boot, or when you log in.
+
+* webapp
+
+  Opens a web browser, viewing the git-annex assistant's web app.
+  (If the assistant is not already running, it will be automatically started.)
 
 # REPOSITORY SETUP COMMANDS
 

doc/install.mdwn

@@ -48,6 +48,8 @@ To build and use git-annex, you will need:
     (optional; Linux only)
   * [dbus](http://hackage.haskell.org/package/dbus)
     (optional)
+  * [yesod](http://hackage.haskell.org/package/yesod)
+    (optional; for webapp)
 * Shell commands
   * [git](http://git-scm.com/)
   * [uuid](http://www.ossp.org/pkg/lib/uuid/)