Update CHANGELOG with 0.6.0 changes. (open-telemetry#685)

Update CHANGELOG with 0.4.0/0.5.0 (open-telemetry#633)

Version 0.4.0

Update CHANGELOG for 0.3.0 (open-telemetry#481)

Use Absolute(true/false) vs NonNegative/Signed (open-telemetry#312)

Add specs on Propagator API. (open-telemetry#147)

* Add specs on Propagator API.

Fixes open-telemetry#72.

* Fix a bracket.

* Update TOC.