Add docs for handlers, app handler

docs/narr/handlers/arch.rst

@@ -0,0 +1,31 @@
+
+Architecture
+============
+
+Handlers are similar to a "plugin" concept in that multiple handlers
+may be installed e.g. by different packages.  But whereas one might
+"enable" multiple plugins, only *one* handler may be used, for a given
+purpose.
+
+There can be many "types" of handlers; each is responsible for a
+certain aspect of the overall app.  So it can be thought of as,
+"*Which* plugin should *handle* this aspect of the app?"
+
+
+What a Handler Does
+-------------------
+
+Each type of handler does something different.  For instance there
+might be an "auth handler" responsible for authenticating user
+credentials.
+
+The app itself will define the need for a handler.  For instance if a
+user login mechanism is needed, then the app might define the "auth
+handler" (e.g. ``AuthHandler``) base class, and add a way to locate
+and use it at runtime.
+
+Other packages might then also define "auth handlers" derived from the
+base class, and perhaps a way for the app to locate them as well.
+
+The app should probably have a way for the "choice" of auth handler to
+be configurable, and possibly expose this choice via admin UI.