Update README with project information

Author: weavejester

README.md

@@ -1,10 +1,54 @@
 # Duct Handler [![Build Status](https://github.com/duct-framework/handler/actions/workflows/test.yml/badge.svg)](https://github.com/duct-framework/handler/actions/workflows/test.yml)
 
-A Clojure library designed to ... well, that part is up to you.
+A library that provides common [Ring][] handlers for the [Duct][]
+framework.
+
+[ring]: https://github.com/ring-clojure/ring
+[duct]: https://github.com/duct-framework/duct
+
+## Installation
+
+Add the following dependency to your deps.edn file:
+
+    org.duct-framework/handler {:mvn/version "0.1.0-SNAPSHOT"}
+
+Or to your Leiningen project file:
+
+    [org.duct-framework/handler "0.1.0-SNAPSHOT"]
 
 ## Usage
 
-FIXME
+This library provides three [Integrant][] keys that initiate into Ring
+handlers.
+
+`:duct.handler/static` creates a handler that always returns the same
+response map. The value of this key is the response map itself.
+
+`:duct.handler/file` and `:duct.handler/resource` are handlers that
+return files or resources. They take two options:
+
+- `:paths` - a map of paths strings to an option map
+- `:not-found` - a response map to be returned if no path matches
+
+If the `:not-found` option is not supplied, `nil` is returned from the
+handler (which can be useful for trying multiple handlers to see which
+matches).
+
+The options mapped from each path go directly to the
+[ring.util.reponse/file-response][file-resp] and
+[ring.util.response/resource-response][res-resp] functions.
+
+For example:
+
+```edn
+{:duct.handler/file
+ {"/" {:root "public"}
+  "/js" {:root "target/js"}}}
+```
+
+[integrant]: https://github.com/weavejester/integrant
+[file-resp]: https://ring-clojure.github.io/ring/ring.util.response.html#var-file-response
+[res-resp]: https://ring-clojure.github.io/ring/ring.util.response.html#var-resource-response
 
 ## License