Add section on how to add attributes to spans

[pjanotti]

We are missing information on how to add attributes to spans. The issue was prompted by this discussion on gitter:

Nick Chappell @nickchappell 12:12
is it a known bug or issue that span attributes show up as "logs" in the Jaeger UI and not as tags?
(using the OC Go library)

Paulo Janotti @pjanotti 14:24
@nickchappell TimedEvents (Annotations and MessageEvent) will show as logs, but, Attributes should show as tags.

Nick Chappell @nickchappell 16:30
@pjanotti googling those terms, they come up for the OC Java library, but there's nothing in the Go lib for them
my code is following the example on https://opencensus.io/tracing/span/time_events/annotation/#0

Paulo Janotti @pjanotti 16:54
@nickchappell the code is here https://github.com/census-ecosystem/opencensus-go-exporter-jaeger/blob/master/jaeger.go#L203 the link that you sent indicates that an annotation, which is a time event and as such goes as log. Use https://github.com/census-instrumentation/opencensus-go/blob/f6172e422374225c6748bd2a9c872e88c2eb43d4/trace/trace.go#L401
Of course there is a gap in the docs…