* Add semantic conventions for Resource labels
This document previously contained semantic conventions for Spans
but was missing conventions for Resources. I copied Resource conventions
from OpenCensus project [1] and also added Service conventions which
were a TODO in OpenCensus conventions.
[1] - https://github.com/census-instrumentation/opencensus-specs/blob/master/resource/StandardResources.md
* Add namespace field and fix linting issues
* Split Resource conventions into a separate document
* Clarify `service.id` label and Required column meaning
* Add further clarifications
* Fix build
* Relax service.id requirements
* Add clarification that labels can be used generally to describe a resource
* Remove requirement for global uniqueness of service.id
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