Add more information and an example to the README

README

@@ -2,3 +2,48 @@
 
 This Guile library provides instrumentation code intended to be used
 with the Prometheus time series service.
+
+Counter, gauge and histogram metric types are supported.
+
+This client library is still in early development, and there will be
+probably be breaking changes in the future, prior to the first stable
+release.
+
+Some information on how to use this library can be found in the
+docstrings of exported procedures in the prometheus.scm file.
+
+** Web service example
+
+The following example demonstrates a Guile web server, with some
+instrumentation for the number of requests. Each time a request is
+handled, the metric is instrumented. For the /metrics path, the
+response is the metrics and their values. This is intended to be
+scraped by Prometheus so that it can retrieve and record the metric
+values.
+
+#+BEGIN_SRC scheme
+(use-modules (web server)
+             (web request)
+             (web response)
+             (web uri)
+             (prometheus))
+
+(define my-registry (make-metrics-registry #:namespace "exampleapp"))
+
+(define request-counter (make-counter-metric my-registry
+                                             "requests_count"))
+
+(define (request-handler request body)
+  (metric-increment request-counter)
+
+  (if (equal? (split-and-decode-uri-path (uri-path (request-uri request)))
+              '("metrics"))
+      (values '((content-type . (text/plain)))
+              (lambda (port)
+                (write-metrics my-registry port)))
+      (values '((content-type . (text/plain)))
+              (lambda (port)
+                (display "Incrementing metric\n" port)))))
+
+(run-server request-handler)
+#+END_SRC