Opentelemetry-dotnet: Establish release cycles in both this and contrib repo

[dbarbashov]

Right now there is no formal release cycle and no actual information about planned features/milestones and overall project status. There is no way for contributors to know when to expect next release.
From gitter: as @CodeBlanch said right now packages are pushed on demand. So we need to establish release cycles with milestones. About after beta as @cijothomas said.

[cijothomas]

I'll cleanup milestones in this repo first, then do the contrib repo. Both will happen after the current beta release.

[cijothomas]

This repo is cleaned up and milestones are currently set. Issues for next beta is assigned as well. Closing this.
Will need something similar for -contrib repo.