Add a readme for the Prometheus exporter. (#91)

* Add a readme for the Prometheus exporter.

* Update README.md

nit

* Update README.md

OpenCapitals

* Clarify registration order.

Author: Ian Sturdy

opencensus/exporters/stats/prometheus/README.md

@@ -0,0 +1,58 @@
+# OpenCensus Prometheus Stats Exporter
+
+The OpenCensus Prometheus Stats Exporter is a stats exporter that exposes stats
+to the
+[Prometheus cpp client library](https://github.com/jupp0r/prometheus-cpp).
+
+## Quickstart
+
+### Prerequisites
+
+The Prometheus exporter exposes stats on an HTTP endpoint that is scraped by
+a Prometheus server. To install and run the Prometheus server, follow
+the instructions [here](https://prometheus.io/docs/introduction/first_steps/).
+
+### Register the exporter
+
+#### Using the Prometheus client library's exposer
+
+Register the Exposer on a selected port, and then register the OpenCensus
+Prometheus exporter with that exposer. This may be done either before or after
+views are registered for export and data is recorded, but stats will not be
+available to Prometheus until the exporter is registered.
+
+```c++
+  auto exporter =
+      std::make_shared<opencensus::exporters::stats::PrometheusExporter>();
+
+  // Expose a Prometheus endpoint.
+  prometheus::Exposer exposer("127.0.0.1:8080");
+  exposer.RegisterCollectable(exporter);
+```
+
+See also the Prometheus client library's
+[instructions](https://github.com/jupp0r/prometheus-cpp#usage).
+
+#### Using a custom exposer
+
+If your application already runs an HTTP server, you may want to expose
+Prometheus stats on a page by that server rather than through the default
+exposer. To do this, create an instance of the `PrometheusExporter` and then
+call `Collect()` to retrieve the latest stats, which can be serialized with the
+tools in the Prometheus client library:
+
+```c++
+#include "metrics.pb.h"
+#include "opencensus/exporters/stats/prometheus/prometheus_exporter.h"
+#include "prometheus/text_serializer.h"
+
+...
+
+opencensus::exporters::stats::PrometheusExporter exporter;
+prometheus::TextSerializer::TextSerializer serializer;
+
+const std::vector<io::prometheus::client::MetricFamily> metrics =
+    exporter.Collect();
+const std::string formatted_metrics = serializer.Serialize(metrics);
+
+```