This codebase should include a 'spec compliance matrix'

[dustinlessard-wf]

Is your feature request related to a problem? Please describe. We do not yet track the level of compliance to the spec inherent to this codebase. We should include a readme, similar to https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md so that we can communicate the current state and to aid us in our development.

Additionally, we should include a section on the readme that indicates 'Feature Status', similar to https://github.com/open-telemetry/opentelemetry-js#feature-status