Add and improve documentation around exposition.

content/docs/instrumenting/clientlibs.md

 ---
-title: Start
+title: Client libraries
 sort_rank: 1
 ---
 
-# Instrumenting your code
+# Client libraries
 
 If you want to monitor services which do not have existing Prometheus
 instrumentation, you will need to instrument your application's code via one of
@@ -23,3 +23,8 @@ HTTP endpoint on your application’s instance:
 
 When Prometheus scrapes your instance's HTTP endpoint, the client library
 sends the current state of all tracked metrics to the server.
+
+If no client library is available for your language, or you want to avoid
+dependencies on a client library, you may also implement one of the supported
+[exposition formats](/docs/instrumenting/exposition_formats) yourself to
+expose metrics.

content/docs/instrumenting/exposition_formats.md

+---
+title: Exposition formats
+sort_rank: 2
+---
+
+# Exposition formats
+
+Prometheus implements two different transfer formats which clients may use to
+expose metrics to a Prometheus server: a simple text-based format and a more
+efficient and robust protocol-buffer format. Prometheus servers and clients
+use [content negotation](http://en.wikipedia.org/wiki/Content_negotiation)
+to establish the actual format to use. A server will prefer receiving the
+protocol-buffer format, and will fall back to the text-based format if the
+client does not support the former.
+
+For details on each format, see the [Client Data Exposition Format](https://docs.google.com/document/d/1ZjyKiKxZV83VI9ZKAXRGKaUKK2BIWCT7oiGBKDBpjEY/edit?usp=sharing) document.