link example when the config file is first named in the README (#6294)

Signed-off-by: Sally Lehman <slehman@ripple.com>

README.md

@@ -75,6 +75,8 @@ read its web assets from local filesystem directories under `web/ui/static` and
 from the root of the cloned repository. Note also that these directories do not include the
 new experimental React UI unless it has been built explicitly using `make assets` or `make build`.
 
+An example of the above configuration file can be found [here.](https://github.com/prometheus/prometheus/blob/master/documentation/examples/prometheus.yml)
+
 You can also clone the repository yourself and build using `make build`, which will compile in
 the web assets so that Prometheus can be run from anywhere: