update README with non-deprecated promhttp Handler (#21)
update README with non-deprecated promhttp Handler (#21)

The `http.Handler()` is deprecated per [golang client http.go comment](https://github.com/prometheus/client_golang/blob/master/prometheus/http.go#L63-L66):
```
// Deprecated: Please note the issues described in the doc comment of
// InstrumentHandler. You might want to consider using promhttp.Handler instead
// (which is not instrumented, but can be instrumented with the tooling provided
// in package promhttp).
```
Update README to recommend non-deprecated [Handler from promhttp](https://github.com/prometheus/client_golang/blob/master/prometheus/promhttp/http.go#L77).
grpc-ecosystem/go-grpc-prometheus,grpc-ecosystem/go-grpc-prometheus,mwitkow/go-grpc-prometheus