Add documentation for how to export exemplars

Expand our exemplar documentation to include information for how to use
the appropriate exposition code and mention the feature flag required in
Prometheus to ingest exemplars.

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

Author: csmarchbanks

README.md

@@ -252,6 +252,20 @@ h = Histogram('request_latency_seconds', 'Description of histogram')
 h.observe(4.7, {'trace_id': 'abc123'})
 ```
 
+Exemplars are only rendered in the OpenMetrics exposition format. If using the
+HTTP server or apps in this library, content negotiation can be used to specify
+OpenMetrics (which is done by default in Prometheus). Otherwise it will be
+necessary to use `generate_latest` from
+`prometheus_client.openmetrics.exposition` to view exemplars.
+
+To view exemplars in Prometheus it is also necessary to enable the the
+exemplar-storage feature flag:
+```
+--enable-feature=exemplar-storage
+```
+Additional information is available in [the Prometheus
+documentation](https://prometheus.io/docs/prometheus/latest/feature_flags/#exemplars-storage).
+
 ### Disabling `_created` metrics
 
 By default counters, histograms, and summaries export an additional series