[repo] Add project READMEs to packages

We implemented this over on the main repo: https://github.com/open-telemetry/opentelemetry-dotnet/pull/5885

Probably a good thing to do here on contrib too. The code can be taken and applied pretty mush as-is. Only difference here is there is no "RELEASENOTES.md" file so that bit should be removed. For "Release Notes" tab we should just include CHANGELOG link.