Update CONTRIBUTING.md with internal examples

[HudsonHumphries]

In PR 97 I added a Useful Resources section in CONTRIBUTING.md that generally point outwards to other Open Telemetry projects and their examples. This Issue will serve as a reminder to update these resources as examples and exporters are added to the C++ repository.

[lalitb]

Closing this. as the examples and exporters have their own README.md in corresponding folders.