Clean up Contributing.md (#655)
Clean up Contributing.md (#655)

* clean up contributing doc

* fixed a missing period

* clarify tags

* minor tweak

* address comments

* Update CONTRIBUTING.md

Co-authored-by: Sergey Kanzhelev <dd2a78e617335ac05535a01af404ce3eb19c30fd@live.com>

* Update CONTRIBUTING.md

Co-authored-by: Sergey Kanzhelev <dd2a78e617335ac05535a01af404ce3eb19c30fd@live.com>

Co-authored-by: Sergey Kanzhelev <dd2a78e617335ac05535a01af404ce3eb19c30fd@live.com>
open-telemetry/opentelemetry-dotnet,open-telemetry/opentelemetry-dotnet,open-telemetry/opentelemetry-dotnet