Provide definitions of terms

[jeanbza]

It would help tremendously to define what a trace, a span, a measurement, view, aggregation, stat, and any other technical terms mean. I think most people have a sense, but putting it on paper would clear up a lot of confusion.

[rakyll]

We will have a new website soon with all these concepts explained soon. For now, you can refer to the specs for tracing and tags, see http://github.com/census-instrumentation/opencensus-specs.

[jeanbza]

Ah! Fantastic. Thanks @rakyll