webui: add documentation

services/webui/README.md

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+# Service: webui
+
+This service provides an HTTP API and a corresponding cross-platform
+webui (written in ember.js) for the core Irdest services.
+
+To use this UI, consult the list of [clients]() that ship this
+front-end.  To work on the front end, see the
+[`./develop.sh`](./develop.sh) script to live-load changes you make to
+this directory.