all: add a section describing the resource entity

[odeke-em]

Resources just got added to the spec in https://github.com/census-instrumentation/opencensus-specs/pull/162 with a reference implementation in Go in https://github.com/census-instrumentation/opencensus-go/pull/878 and then naturally into the data model in https://github.com/census-instrumentation/opencensus-proto/pull/137 all just a few hours ago.

We'll need to document them on the website too.