README.md: fixes minor documentation errors (#16)
README.md: fixes minor documentation errors (#16)

* README.md: fixes minor documentation errors

including formatting issues and code example errors.

Also added a couple of TODO items to clarify the documentations
that can be easily fixed once the spec for stats is published.
census-instrumentation/opencensus-go,census-instrumentation/opencensus-go,census-instrumentation/opencensus-go